Valkyries are pretty good for grinding quickly, but...yeah, not a class I'd use for running through the game either. I'd say just max out a load of jobs for everyone and experiment, but I enjoy doing that sort of thing. xD
Ohh, the summoning feature...I had a couple of good players as a result of that, although I rarely used them outside of the terrifying DLC boss battles. Seriously, that bird/fish thing scares the hell out of me, and the creepy child vocal theme doesn't help matters either. xD