I wouldn't say many gimmicks was a bad thing, unless it's made obvious that the same guy has had several gimmicks.
For example, Mick Foley. What exactly was the point of having 3 gimmicks in WWE? Dude Love was something he never wanted to do in his professional career. Cactus Jack was his WCW/ECW gimmick and Mankind was his fledgling WWE gimmick. He was over as Mankind, to a high degree. Why did they need to have his other gimmicks come into play? What benefit did it give anyone? At the end of the day he may as well have been 'Mick Foley, the crazy fucker'. It's still the same guy taking the same ridiculous life shortening bumps, and giving pure gold on the microphone, except he occassionally changed his ring attire and catchphrase, and sometimes danced like it was the 60's and sometimes wore a sock on his hand. What else? Would HIAC have been any less of an iconic moment if Taker had thrown Dude Love off the Cell? Would his retirement match at NWO have been any less emotional if HHH had pinned Mankind? NO! The only thing the world would have missed out on was Mick Foley in the '98 Royal Rumble 3 times. Big whoop!
In other instances, it can indeed help a guy become a superstar, provided they don't mention the fact that he was once someone else. Did they mention that Rikishi used to be the Sultan? Did they make a big song and dance about Festus being the fake Kane? Are they going to mention that Umaga used to be Jamal in 3MW? Of course not. Because it wouldn't help anyone, and would probably only make things worse for that superstar.
In Rock, Austin and HHH's case it's different. They are characters that have developed.
Rocky Maivia started getting pissed off at the fans for jeering him constantly and dropped the happy go lucky mid card guy look, joined some pissed off black guys and became The Rock who wasn't going to take shit from anyone anymore.
Stunning Steve Austin doesn't need mentioning, it was a shit WCW gimmick and isn't relevant to his WWE career. The Ringmaster was Ted Dibiase's puppet and once Dibiase was 'forced to leave' it meant Steve Austin had to go it alone, and became Stone Cold. I didn't watch WWE back then, so someone tell me, did he leave and re-debut as Stone Cold, or did he just turn up one day and start flipping the bird and drinking beer?
HHH the blue blooded asshole made a couple of high profile friends and became just an asshole. He didn't leave for months and come back as someone else did he? He just changed his mannerisms and abreviated his name to something cooler.
In Kane's case it hasn't helped him at all and i don't feel i need say anything else about that.
Basically what i'm saying is, when a guy stops wearing genie pants and decides to carry a styrofoam head around, it's not going to help him get over if you remind people that 'Hey, The Snowman Al Snow was once Avatar', because then people will say 'Really? Avatar was shit, so therefore Al Snow will probably be shit too,' and some 60-75% of those people who learn that this new guy is someone they used to see, will switch off and not give him the time of day, and won't notice that the Head gimmick was actually pretty fun and a laugh to watch.
However, not changing your gimmick can be the death of you professionally sometimes. Gangrel's teeth will prevent him from ever ditching his vampire gimmick, but that's his real lifestyle. Perhaps if the Mean Street Posse had come up with some new ideas, they wouldn't have been reduced to jobber status and ditched. If Steve Blackman, Lance Storm and Dan Sevren had been a little more up beat, they'd have been given better runs in WWE. Kurrgan got over didn't he? He was 1997's equivalent of Kozlov now. Then he joined the Oddities and people liked him.
Other guys change gimmicks and the world stops caring. I bet Headbanger Mosh wishes he'd never become Chaz, because he was shortly unemployed after that gimmick change. Kama Mustafa got a few cheers as the Godfather, and nothing else. Bull Buchanan was B2 for like 3 weeks before they ditched him.
Could Muhammed Hassan have become a big time player if he'd dropped the pissed off Iraqi and become someone else? Did they HAVE to make Taker bury his career by powerbombing him off the stage and never mentioning him again? Or could they have given him a mask and some new tights and say 'hear you go, try that on for size and pray no one blows up a bus wearing a similar outfit this time around'
Sorry i've babbled a lot, here's my main point. Giving a guy a lot of gimmicks but letting the World know that three guys are in fact the same person ruins the mystique and aura, not only of all three gimmicks, but also the wrestler himsef, and the profession overall in one foul swoop. For me it's the same with shoot interviews and guys who have tantrums and shout at the bookers on TV when they aren't scripted to i.e every instance that Bret Hart got in Vince's face on Raw. Here's a traditionalist with absolute dedication to the business and its inner workings, and he goes and pisses all over kayfabe by shouting directly at Vince in front of the world because he doesn't appreciate being booked to look weak.
Vince McMahon kinda ruined HHH's character a little by calling him Paul on live TV. The audience is supposed to believe his name is ACTUALLY Hunter Hurst Helmsley, and in two seconds Vince McMahon ruined that facade for ANY wrestling fan that is committed to believing the lie. The Undertaker's wife did the same thing when she called him Mark during the DDP stalker angle, which is probably why they nver gave her a microphone again after that.
Multiple gimmicks? An ok idea if done properly, totally counter-productive for everybody involved with that company when they try to mix real life with kayfabe in the same instance imo. How big a joke would Kane be if all the new marks knew he was once Jerry Lawler's dentist and was of absolutely no relation to the Undertaker in any regard? (and yes, i'm sure people are going to say he's a joke now, but it'd be a fuck load worse if they mentioned his previous alter egos)