At the risk of rehashing a few arguments that have already been presented in this thread, I'd like to present a couple of thoughts of my own because...well, just because.
On Aldaron's Proposal/the Drizzle + Swift Swim ban: As Aldaron made clear during the discussions at the time of the vote on this particular ban, DrizzleSwim was designed to be a one-time deal that was only enacted because of the sheer volume of the problem set before us at the time. The issue with Drizzle + Swift Swim encompassed more than just one Pokemon (as is the case with Greninja and Protean); it encompassed an entire playstyle and multiple Pokemon that were broken under it. We were stuck between a rock and a hard place because we had three simple ban options, each with a certain amount of unfortunate collateral damage that would occur as a result. Banning Drizzle would have eliminated an entire subset of Rain-based playstyles and could have very well led to an unbalance in the weather game, causing an increased need for bans regarding Sun (and possibly even Sand). Banning Swift Swim would have eliminated the possibility of non-Drizzle Swift Swim abuse (rare but viable). More importantly, it would have soft-banned several Pokemon that only had Swift Swim as an ability at the time (I believe Floatzel was one such case). The Pokemon in question may have not been very relevant in OU to begin with, but essentially banning Pokemon that have no reason to be banned goes against our ban philosophy. The third option would have been to ban the broken abusers, but there were already a pretty solid number of Swift Swim users that were considered to be broken. Doing so would have resulted in a pretty large number of Pokemon bans all focused around this one central breaking factor, not to mention the possibility of more broken Swift Swim users rising in usage that we may have not noticed before due to the competition among such Pokemon. Drizzle + Swift Swim was a drastic answer to a drastic situation that encompassed entire playstyles and left us with no simple solutions.
On a side note, there are actually quite a few BW players that think that we should have gone ahead and banned Drizzle early in the generation so as to save ourselves some trouble later into BW, so it might not be the best idea to use DrizzleSwim as an argument. Just a thought.
On the Sand Veil ban and collateral damage on lower tier Pokemon: Again, there is a major difference between banning Protean for Greninja's sake and banning Sand Veil during the BW2 era. The first major reason is that Sand Veil (and Snow Cloak by extension) were considered to be uncompetitive abilities due to the luck factor that they brought to the game. Not only that, but we also had a precedent for such a ban due to our current Evasion Clause. The clause was designed to prevent factors in games that would introduce too much a luck due to evasion manipulation, and Sand Veil and Snow Cloak, despite being a bit more situational, did fall into this category. Another factor involved in this decision was the numerous Pokemon that became a problem thanks to Sand Veil abuse. Garchomp was banned primarily because of how effectively it abused Sand Veil and used those misses to completely destroy teams, even if they had a pretty solid check or counter to it. Although it was the only one that was banned, it wasn't the only issue. Gliscor had gotten a lot of attention at the time because it was able to pull off a Garchomp-esque SubSD set that could abuse a single miss to sweep teams with its STAB Acrobatics and Earthquake. There were even stories of people using Cacturne successfully due to its ability to sweep if it got lucky enough under Sand. In addition, banning the abilities allowed us to release Garchomp from Ubers in the hope that it would be a positive contributor to the OU metagame. Now, I am NOT saying that the Sand Veil ban was all about getting Garchomp out of Ubers, as is a common misconception. I am saying, however, that it was a nice little perk to a ban that was mostly centered around the uncompetitive aspect of Sand Veil. Given the luck factor introduced by Sand Veil, the precedent we already had with Evasion Clause for banning evasion-based aspects of the game, the multiple Pokemon that were able to abuse that luck factor successfully (granted, some more so than others), and the minor but positive factor of having Garchomp back in OU, it was decided that the Sand Veil ban would be a wise decision.
As far as collateral damage goes, you have to remember that the Sand Veil ban didn't do too much to lower-tier Pokemon. All of them had other abilities that they could use, and Sand Veil itself was basically a blank ability in lower tier play due to the fact that there were no Sand Stream users and manual Sandstorm was a waste of time more often than not. In addition, unless I'm forgetting something, the only Pokemon that actually suffered from move illegalities with its alternate ability was Cacturne (the moves in question were Encore and Bullet Seed, if I'm not mistaken). However, even then, Cacturne still had viable alternatives to its lost moves (Substitute and Seed Bomb were the main ones brought up as replacements), so it wasn't a huge hit to its viability. Banning Protean, by contrast, would have a much more substantial impact on a certain lower-tier Pokemon: Kecleon. Once it got Protean and all the great tutor moves in ORAS, Kecleon actually turned out to be a pretty solid choice in lower tier play. Banning Protean would rob Kecleon of its main niche and would cause it to fall into the mediocrity that it suffered for the last few generations, as opposed to Cacturne, who just lost a couple of key moves. It would also destroy any chance that Frogadier would have of being viable at all in NU or lower (it's currently C Rank in NU and B Rank in PU). Of course, we must keep in mind that collateral damage involving Pokemon in lower tiers (much less Other Metagames like PU, even if it is a mere extension of our standard tiers) is an extremely minuscule concern when we're discussing OU policy and will generally mean virtually nothing except in extreme circumstances (i.e. unintentionally soft-banning several non-banworthy lower tier Pokemon had we implemented a Swift Swim ban in BW). However, in the event that you do want to discuss collateral damage, I'd say that a Protean ban would have a larger impact than the Sand Veil ban.
On Greninja and Protean: So where does that leave use with Greninja and Protean? When compared to these aforementioned bans, a Protean ban is nowhere near as necessary or justifiable. Unlike DrizzleSwim, a Protean ban would only concern a single Pokemon rather than entire playstyles, and the more simple and standard ban (just banning Greninja) has nowhere near as many negative side effects as the simple ban options did during the time of the DrizzleSwim test. The only real negative is the inability to use Greninja in OU, which is no different from many other OU Pokemon bans and is not high on our list of concerns. There are also several differences when compared to the Sand Veil ban. Unlike Sand Veil, Protean is not an inherently uncompetitive ability, a factor which we have always tried to limit as much as reasonably possible through our tiering decisions. It also does not have a precedent ban regarding its primary effect like we had with the Evasion Clause and Sand Veil. Moreover, Greninja is the only Protean user that is even a slight problem in the current metagame, whereas Sand Veil helped break Garchomp while causing other Pokemon to be problems at times (Gliscor, again, was the big one here). Lastly, even in the case of collateral damage for lower tier Pokemon (hardly a concern), it can be argued that a Protean ban would have a more significant effect on its lower-tier users than the Sand Veil ban did. A more comparable example would be Speed Boost + Blaziken, which has already been discussed in both this thread and many, many times elsewhere on this forum. The bottom line regarding that argument is that we have decided over and over that simple bans regarding Pokemon such as Blaziken and BW Excadrill are far more desirable than complex bans involving their powerful abilities due to the more simple nature of the bans and the can of worms that we would open up if we did implement such complex bans. The best decision to make in this case is to simply ban Greninja, and I don't think that past bans such as DrizzleSwim and Sand Veil are enough to justify a Protean ban.
On a side note, I'm kinda glad to have a chance to articulate my thoughts on ban precedents and complex bans (mainly regarding ideas like Drizzle + Swift Swim, Sand Veil / Snow Cloak, and Speed Boost + Blaziken), because I don't think I've ever done that here.
