I habitually create accounts and then delete them; I've done this for years. The most accounts I've had at one time is four, and three of those were created for specific purposes, and weren't really 'me' as such. In all honesty, I've stopped doing that now because it gets ridiculously expensive and I can't help but mess around and spend money on each avatar to make it look the way I want it to, and of course that kind of thing is endless. So I'm down to one account for now, and for the forseeable future.
I have no problem with alts, obviously, and as someone else mentioned, you can tell your closest friends about your other avatars, and it does create a level of trust between those who know a particular avatar is yours. I see it as just a normal part of SL ('normal' being a relative term in SL).