Yeah, I have 8, but only really play 4 at this point. The others were on one of the 'affected' realms that were crashing all the time in December and January. I've abandoned those characters for now and moved to Horde side on Terenas. Terenas is NEVER down.
I understand those realms still have problems. I wouldn't know because my realm is literally never down except for the scheduled maintenance. That switch was the best move I ever made.
The people I know with so many characters generally have 1 main, 2 or 3 secondaries and the rest are all like in the teens. You gain a lot of understanding of how the other classes work when you play them for a few levels. It greatly enhances your group skills when you understand what your groupmates are likely to be doing (if they know what they're doing). I think everyone with a main should make an alt of the other 2 of the 'golden triangle' (DPS class, healer class, tank class) to understand the tools each has for dealing with aggro. It's easy to talk what it's like for a warrior to hold aggro, it's quite another to actually play a warrior in an instance.
It also helps identify weak players in pickup groups much faster, differentiating those who are simply clearing hall trash quickly but are actually good players, from the truly careless who are bound to cause a wipe.