You and I have gone back and forth in this thread many times, and at this point we can agree to disagree. I'll agree that those who PvP a lot will see more use out of certain abilities than those who PvE (and of course the other way). Shadow-walk is useful and hell I've used on occasion. I will still argue many abilities are there to support bloated mechanics that really add unnecessary complexity to the game and hinder balance (like levels of stealth). I will also argue cooldowns to reset cooldowns is simply bad design and absolutely button bloat. However, there is one flaw here I *MUST* address.
Your assumption about the number of abilities influencing skill-cap is a fallacy in many cases, just not rogue. Just to preface: this is strictly speaking for PvE. WoW's PvP balance is atrocious and basically a giant game of rock-paper-scissors, IMO (you can try to convince me otherwise, but seasons of gladiator breakdowns will prove otherwise how class comp matters more than skill). Lets take a look at a class that is quite similar to rogue in a lot of ways: the retribution paladin. Ever hear of a mod called CLCRet (or CLCInfo, which now can be used for virtually any class)? The issue of button bloat becomes trivialized into a game of simon says. Rogue is simple enough dealing with maintaining buffs/debuffs, but when you start dealing with cooldowns that don't line up and a priority queue involving 5+ abilities, very few people can actually juggle that much information on a GCD-by-GCD basis. Now it has gotten better since WOTLK (when it was really required with 4pc), but it sheds light on a glaring issue with the game. The bloat has grown to the point that very few people can get the information they need from the default UI. Anticipation, Tigereye Brew, all these extra mechanics that simply involve a Nth level resource to watch. On top of that, you have bosses with 5-10 abilities on timers that aren't multiples of each other, so there's no pattern. You HAVE to use a custom UI (the default WoW UI is so terrible they absolutely HAVE to keep supporting custom UIs) to manage all that information with addons like WeakAuras and DBM. So I guess my complaint isn't with the button bloat alone so much as the combination of too many buttons with a UI that doesn't properly handle it. And then I have to spend hours configuring it to provide information in a format that I can handle it. It's not that I CAN'T handle all the buttons in the game, but I do have to work to a point where I can. Perhaps it's just that I prefer twitch-based gameplay, and I'm trying to project that onto WoW.