Bard as subjob doesn't seem to make much sense. You won't be able to equip instruments and would only be able to play one song at a time I believe. So bard would really be a main job only choice.
Lets test that theory...
lets take any class randomly subbed with bard then subbed with anything else:
umm. lets see..how about monk, that isn't a typical combination u see.
Monk / Bard:
(Speaking from full lvl 70/35 example)
You have the great hp and damage from the typical monk you would normally see, with it's chakra and other counter attacks makes it a great fighter. Sub that with bard you now have the available abilities , with one song allows you one buff. what could a monk use to buff itself at thirty fifth level. army's paeon III for example , an incredible help in hp regen. can't claim that doesn't help. Sinewy?? increase in str, guess you don't need those mithkabobs anymore..or infact lets stack them.. 2 str bonus's increase damage output, " Magic Finale" - Cures detrimental magic effects on group" that goes without saying.
ADVANCING MARCH whats better than a attacking monk... a Hasted attacking monk. You get my point though.. there is single songs that will help out any class immensely as well as an entire group as a whole.
I don't think i honestly need to explain what other classes benefit the monk in combination, because any class i list will help the monk in some fashion or another. point being don't assume that becuase your subbing a class that it makes it worthless.