It's not a good thing whatsoever. It's treated as a gimmick, and people will be rolling up a magicians' sleeves to check for secret stuff. Pushy spectators will do whatever they want to. If the thumb-tip were exposed to many people, spectators would be yanking at magicians' thumbs all the time. If this becomes well known enough, another will be made that is mounted in some other fashion or works in a completely different way that will combat this, which is a situation that is utterly ridiculous for a magic trick to be in.
While I do get what you're saying, as the guys above me said, the TT was possibly the worst example you could have used. It's one of the most well known gimmicks outside of the magic world, I think. There's one in like every magic kit ever. And let me tell you, one of the more frequent stories I've heard from other performers is audience members telling them a variation of, "When I was a kid I had a magic kit that did that silk-vanishing trick .. mine used a fake thumb, though. You must use the professional's method."
Look - if you act like shooting fireballs out of your hand is THE trick, then yes, people are going to look for a gimmick. Because you're no giving them anything else to do or focus on. Integrate it into a routine where the fire isn't the point, but a highlight, and you won't run into this problem. This is commonly referred to as 'Being a good performer'.
Also - if a spectator is going for your sleeves, you should drop what you're doing and specifically work on audience management because that is not cool and you are not handling the situation the way it needs to be handled.