The only problem with playing a bunch of classes is gearing them past i91. I would say it's beneficial to play through all the content as one of the 3 roles. It will make you a better player since you know what other roles should be doing and what you yourself should not be. Also depending on the fight you might want to learn both how to Off-tank, main-tank, play as a ranged dps, and play as a melee dps.

I've done almost all content as every role and have decided to main as a tank because it's easier to get party spots. I don't know how some people can stand more than 10+ minute waits in queues. If for some reason I'm not getting instant queues I'll hop on my healer so I rarely play DPS.