Originally I think I started to multi-table pretty much out of boredom. I wasn't a very good player at the time, and single table play was just sort of boring to me. I know that sounds a little silly but it's the truth. I enjoyed donking around, and wanted to do it faster.
By the time I was really putting some effort into my game, I was playing 6-8 tables at a time. Of course, that's too many when you're really trying to pay attention. So ultimately I cut that number in half.
I'm a winning player at my stakes now, and rarely play more than 6 tables at a time. I am capable of playing more, but my play becomes a little more automated than I would like.
I think a degree of automated, robot like play kicks in once you start playing any more than 2, but a little bit of that is ok. To me, it's about finding a nice balance between volume, ability to focus and think, and keeping yourself amused.
I think being able to multi-table is a worth while goal for a player to achieve, but it should never become a bigger priority than thinking about your moves and improving your game.