I've never understood the point of leveling more than 2 max's tbh, takes too much time to gear them up, unless you're a player who enjoys leveling and playing different classes.
It really isn't that much work if you play more than a couple of times per week. In Cataclysm, I had 6 85's with I think 4 of them geared enough for that raid tier with 2 being extremely geared. The only tedious part is the beginning of the gearing experience which consists of running heroics over and over. But once you unlock the lfr on the characters it's just a matter of doing it on each of them once per week.
MoP made gearing more difficult than in Cata, but it's still very manageable.
As for why people do it, there are a number of reasons. The people in the "hardcore" guilds need the alternates. They swap classes in and out, do multiple runs per week. (Which requires another character as the main one will be locked until reset day).
If you want more than 2 professions you need extra characters. And as you said, some people just like having extra classes.
Me personally, I'm a bit of everything. Once your main starts nearing a gear plateau, it makes it a lot more fun to have another character to mess around with. I also used to do multiple runs every week.
I'm working on my 3rd 90 right now. My second one happened quick. Mostly because I leveled my Mage first which I played and raided through every tier in Cataclysm with. Made it to 90 to find out I wanted something new, so I leveled my hunter.