There is another benefit to leveling other classes that I don't think anyone mentioned. (Sorry if I missed it.)

By leveling other classes you can understand how they work and what skills they are using.

In a dungeon you see a swirling puddle of sand on the ground? Is that something a monster used? Or a skill your teammate used? Is it safe to stand in? Or should you avoid it?

By understand what skills other players are using and how they use them, you can work more effectively as a team.