If they are severe I do find them a turn off but like others have said, if you can see them on the profile you have a chance to make an informed decision. As long as profile pics are accurate (ie not airbrushed and not covering everything up with clothes), I don't have a problem. That goes for everything which people might find offputting - excessive tattoos, skin blemishes, stretch marks, whatever.
Basically, I think that if someone is going out of their way to hide some aspect of what they look like, it is indicative that they are aware there is something people might not like and they are relying on the punter's urge for a shag to override his brain when the true appearance is revealed. Chances are that they will provide fairly poor service. I had a girl like that last year (reviewed on here) - looking back at her profile pics, it should have been obvious that she was hiding something (in her case a very saggy 'mummy tummy').