I have always leveled separate classes. I guess I like the challenge of getting all the different skills working together. People usually choose similar classes to keep macros and keybinds to a minimum.

I also like semi-roleplaying and having a mini story that goes with why my characters are together. The triplets/quadruplets/quintuplets thing only works so many times :P

I would also recommend against multiple melee classes unless you are very patient. It is doable. But you will be constantly realigning to always be in range. My Warrior and Paladin have been fun, but any groups from here on out will be a Warrior/Pally/Death Knight with a bunch of casters, or an entire caster group.