BARD/WMGFollow

Yes, in fact the strategy guide lists this as one of the most common choices.

It mentions that as a good choice for a tarutaru, who will then have just enough magic points to be a support healer in a group which already has a main white mage.

Bard/thief is mentioned for someone looking to break from the standard mold.

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.

Decibelle

Eh'??

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.