1) Crossclass skills. These are important.
2) Eventually if you're diligent about getting your weekly loot and tomestones, you'll run out of good pieces for your given class. For example, if you main BRD there's really only 6 pieces of gear you can buy with poetics that are any good for you. 7 if you carboncoat a ring. this means after about 9 weeks, you're done. It's been about 6 months since poe came out. Also, you potentially get 8 pieces of coil loot a week for 8 players, but often (VERY often) pieces will drop for classes that aren't present. Also, often the same piece will drop multiple weeks in a row. One static on my server has nearly gotten enough PLD swords from t13 to gear all their members, while another hasn't gotten any.
3) Crossclass loot. A lot of gear is shared between WAR and PLD, NIN and BRD, DRG and MNK, WHM and SCH, and SMN and BLM. It's MORE efficient to gear up two classes as one character than two classes as two characters. There's even crossover between WAR/PLD and MNK/DRG since a lot of WARs want some DPS accessories.
4) Unlocks. Not having to go through content twice (aside from relics) is a HUGE plus.
5) EXP bonus: Leveling a second class on the same character is faster than leveling a new character because of the exp bonus you get for having a lvl 50 class already.
