The only reason to play more than one class is simply to break up the monotony of one job. Once you get into the post 50 grind for virtually everything to progress, you'll begin to see just how tedious and time consuming it becomes to keep 2 or more characters on par.

I started out doing it on smn, pld, and drg. Then dropped to smn and pld. And now pretty much just keeping smn completely up to date and keeping pld up just enough to be competitive in order to get instant content queues.