I'm boxing 5 different classes right now. It's a great challenge for sure, but you also get great rewards. Couple of bonuses in my group.

4 different damage types: Nature, Frost, Shadow, Holy. Good for mobs like the voidwalkers in Arcatraz, any kind of elemental, etc. No real option for empowering dps by ie. curses, but I must say I would probably not have used curses on anything but bosses. Bosses are the most important mobs, but I feel the synergies from having different classes far outweigh the 10% extra damage from a curse.

Soulstone, Anhk, Divine Intervention are pretty nice to have.

Totems, PW:F, AI, Spirit, imp buff, blessings, PW:S, Earth Elemental, Heroism, Pom, spellsteal, 2x aoe fear, nova and the list goes on and on.

The great thing about 5 different classes is that I have so huge amount of options. Tons of different strategies I can use on bosses. The bad thing is naturally that it's pretty difficult and micro intensive, and will probably take anyone a couple of months to become good at. But leveling 1-70 could take a month or two, so one has plenty of time to practice.