So before playing 90+ games on the LC suspect ladder I was pretty neutral towards a Diglett ban - leaning towards no ban but definitely open towards the suspect. Now I'm fully on board with a Diglett ban!
Pros:
+ Trubbish is now C+ material instead of C+
+ More 30% burn chances so we're more like other tiers
+ Diglett has way too many wins in SPL. Almost a 50% win rate
Cons:
- All of the above
Long story short, getting reqs left a very bad taste in my mouth.
In theory, Diglett is overcentralizing and limits teambuilding, but from what I have seen, this doesn't look to be the case. I think that a big reason for this would be that even though Diglett can limit a Pokemon from a defensive perspective, it often struggles to actually take out what it needs to due to how difficult it is to get in, at least if the Diglett-weak Pokemon is played well. As such, as long as the team isn't overly weak to Diglett the way you wouldn't want a team to be overly weak to Fighting-types or Snivy or even Fletchling, it isn't unbearably limiting. This is demonstrated in the Diglett-less metagame by the fact that pretty much every Pokemon that was good or bad in the current metagame remains that way without Diglett; I can't think of more than a few noteworthy Pokemon that might rise or drop more than a single sub-rank. Things that were badly crippled by Diglett, like Trubbish, are still very difficult to justify using. Pokemon like Ponyta and Chinchou already had the means to get around Diglett, so even though they've improved noticeably, it isn't an enormous jump. Specific Diglett-disliking Pokemon like Magnemite are held back by the increase in their other checks, such as Chinchou.
So with that out of the way, we get into more subjective things - perhaps we should ban Diglett because it's annoying, and perhaps we could say that a metagame without Diglett is more fun. If I was on the fence about this before, I cannot begin to express how strongly I disagree now. Although I mentioned that nothing has jumped entirely from obscurity to viability or vice versa, there have definitely been some metagame shifts. The main Pokemon that have gotten better are those that rely heavily on 30% burn effects - Flame Body Ponyta, Scald Skrelp, Scald Chinchou. The main Pokemon that have gotten worse - Shell Smash Shellder, offensive Vullaby, Foongus - happen to rely on guaranteed or close to guaranteed effects. Now don't get me wrong; Shellder, Vullaby, and Foongus are still fantastic Pokemon. For example, I would argue that Foongus has gone from the top of mid A to the bottom of mid A. Regardless, I found the metagame shifts, however slight, to be incredibly infuriating. The metagame feels more luck-based as a result of the emphasis on burn spam, and I am personally not fond of this at all.
The main Pokemon other than Ponyta/Skrelp/Chinchou that noticeably benefits from a lack of Diglett is Gothita. However, we're talking about whether a Pokemon is more or less fun, and I can't see how Gothita is more fun to play against than Diglett when they both mostly just revenge-kill things and then get forced out afterwards.
Perhaps Diglett isn't overcentralizing or unfun, but just outright broken. After all, cores like floondig and shelldig are very strong on paper. I can't argue against these cores being top notch, but I do not think that they are broken. There are other good cores, but the most popular offensive cores in LC tend to involve strong offensive Pokemon + something to trap a handful of specific Pokemon. Although Diglett is usually considered an overall better Pokemon than, say, Gothita or the various Pursuit trappers, this tends to be a moot point as a member of these cores, when its main use is just to trap what it needs to and let the scary offensive mon handle the rest. In fact, if we're talking purely based on the number of Pokemon a core can break through, OmaGoth and OmaCarv are arguably more threatening. Of course, Diglett being the overall stronger Pokemon is helpful in match-ups where the core isn't likely to do very much, but each specific Diglett core has a number of issues that, in my opinion, prevents the core from being broken. I do not plan to detail them in this post, however.
Please keep in mind that I do not mean to speak badly of people who pushed for a Diglett suspect test, considering how I was in some ways one of them. If anything, this suspect was what really convinced me that Diglett is anything but unhealthy for the metagame.