So far, there's been essentially three main points brought against retaining sleep clause that I'll sum up by quoting below.
Clauses and bans should only exist in Ubers if they would prevent something both overcentralizing and extremely luck-reliant (turns battles into a match of coin flips).
Manaphy seems like a great anti-meta pokemon right now. Nothing screams "fuck you sleep" like Hydration.
In other words, there's niche counter.
"Just use Lum Berry to kill Darkrai as it DVoids."
The first one I'm just going to completely throw out as it is based on a fall premise. These have never been criteria for banning in any tier ever and are just a misinterpretation of previous Ubers suspects. The only criteria that has been set in stone so far are the ones presented here:
http://www.smogon.com/forums/showthread.php?t=66515 Of course, this list isn't perfect so if you can present and defend arguments of criteria in addition to these then they are legitimate. (although, every legitimate reason pretty much falls into one of the charateristics) Btw, "Overcentralization" isn't one of them but I'm not going to cover that now. My point with this is to present what we are considering when deciding a suspects placement.
Okay, now for why I think Sleep clause should be retained. Although I would love to talk about how broken Butterfree's Sleep Shuffling strategy is (TBH, Smeargle does it better but Flutterby is cooler), I'm going to focus primarily on Darkrai as it was the only sleep abuser that was a legitimate threat in normal Ubers so it is the most consistent as it doesn't rely entirely on Sleep to function. If I were to go into a tourny without Sleep Clause, I'd feel a lot safer working around a Darkrai than anything else since it would be the least reliant on team match-up.
Let's take a quick look at Darkrai in normal Ubers for a second. Darkrai has an excellent speed stat that is difficult to outrun without the use of a Choice Scarf. It has a monster Special Attack stat with a good STAB in Dark Pulse and near perfect coverage with FBlast. (the things that resist Dark/Fight don't see play in ubers) It also has the rare Nasty Plot which allows it to pump up that raw power to wall breaking levels. I'm only stating the obvious here to make the point that, regardless of its access to Dark Void, Darkrai is a viable Pokemon in Ubers. So what does Dark Void bring it? Well, for one thing it eases setup which separates Darkrai from other fragile setup sweepers (like Kabutops) in that it isn't reliant on prediction, team support or profiting from a limited pool of Pokemon to find that free turn. Secondly, it allows Darkrai to instantly eliminate a threat thanks to the harsh sleep mechanics in Gen 5 which can be useful to function as an anti-lead or to pave the way for a teammates sweep. Lastly, Dark Void allows Darkrai
to bypass its own checks/counters. This is the point I'm going to focus on the most although keep in mind the other two as they are important as well. The key bit I want you to take away from this is that there are no true counters to Darkrai in a sleep clauseless metagame and even checks struggle to function beyond a one time stop.
So what about Lum Berry, Sleep Talk, etc? These are tried and true sets that have foiled many a Darkrai sweeps on the ladder.
This is where I want to bring up the point of the ladder being trash. A Darkrai that falls to these very temporary checks is a Darkrai that is in the hands of a poor player and/or used on a weak team. (ignoring instances of hax/team matchup) Again, the ability to bypass its own checks/counters without limits makes Darkrai the strongest win condition in nearly every situation (and in cases where it isn't keep in mind my second point). Just as leading or trying to KO an otherwise beatable healthy Eggy(Lax/Lapras/etc) with your Tauros in RBY is a poor playing, trying to sweep a team with Darkrai without scouting for and/or weakning potential Lum Berry/Sleep Talk/etc. sets is poor play and a horrible waste of a win condition. Darkrai has the gift of great speed and to power to immediately pose a threat to many Pokemon that forcing a switch to these temporary checks/counters is an easy task. There is little need to be greedy and recklessly refuse to switch in a blind attempt to fling Dark Voids at every member of the opposing team. Basically, a smart Darkrai should be using hit and run tactics early/mid-game so that it has no difficulties in cleaning late game.
For example, a team with Lum Ekiller, RestTalk ScarfSect and SpdefOgre has multiple checks/counters to Darkrai. However, none of these enjoy directly switching into Darkrai as a sleep ruins Lum Berrys/Rests and Dark Pulse + Nightmare (previous sleep for Ogre/Sect) + Hazards will quickly wear down these checks/counters. This forces the player to either throw a team member to bring these sets in safely or attempt risky prediction that is never free even when performed perfectly. (except for sleep on already sleeping mons) This places a lot of pressure on the other player to deal with Darkrai while the user of Darkrai has fairly easy calls to make when given the opportunity. On top of this, none of these threats are strong enough to completely sweep competent teams (at least on the level of Darkrai) which means that even once brought in they fail to apply the same level of pressure they just fought/sacrificed to deal with.
So then the trick is to deny Darkrai free turns with (nearly) every member of the team? This is easier said than done as 125 base speed goes a long way and he isn't THAT fragile (252+ Atk Life Orb Rayquaza ExtremeSpeed vs. 4 HP / 0 Def Darkrai: 152-179 (53.9 - 63.47%) -- guaranteed 2HKO) It also drastically reduces flexibility in team building which makes it much easier for the other 5 members of Darkrai's team to prepare for its checks/counters.
Lastly, there are those Pokemon that are serve as much more sturdy switch-ins (as far as sleep is concerned) to Darkrai like Manaphy or Heracross. First of all, almost all (actually, I think all of them) are otherwise niche and ineffective sets and their existence says little about the fact that unlimited sleep is broken. Secondly, these Pokemon are far from impenetrable as Darkrai's Nasty Plot and their limited bulk opens the door for Darkrai to muscle past these as well. Finally and most importantly, the general mediocrity of these sets further limits teambuilding and makes it very easy for the Darkrai user to exploit as free switch-ins to other threats (whether direct or double). As shown in my earlier replay, the opposing Noctowl and my lack of hazards was still insufficient as it gave Terrakion and others multiple opportunities to switch in and wear down the opposing team. This forced my opponent to take gambles to prevent this exploitation which were brutally punished by timely Dark Voids.
So what does all this say? The lack of sleep clause hurts the metagame when considering the characteristics of a desirable metagame (which were linked earlier in this post). Variety in Ubers is hurt as threats like Darkrai heavily encourage the use of specific threats and massive offensive pressure to deny it free turns. (thus destroying a playstyle like Stall or even defensive balance) Balance itself is a characteristic that is violated as Darkrai (amongst other sleep abusers) poses a pressure on the opposing player that drastically outweighs what any other Pokemon can do (please read the post before saying Ekiller/Ogre). Skilled play, another characteristic, is less present as it requires very little to thought to spam DVoid/Dpulse on switches (as well as scouted sets). Lol, luck, lol.