I think it depends on the player, personally I can't handle more than 2 at a time, I just feel like I'm not putting forth my best effort playing >1. I play occasionally and it's not my main source of poker income/playing time(mostly play S&G's) so I don't mind having a bad hourly rate or playing for 3 hours without cashing.. I think if you primarily play MTT's though, you'll get used to more tables and it will of course increase your chances of winning one. With such a high variance, it makes sense to play >1 since if you bust, you still have 3 more that are in progress. Plus, although they run frequently, MTT's don't run all the time, so there will be a lot of overlap and missed games if playing one.

Ultimately, just player dependent, do what you feel comfortable with.