A whm sub for a Bard is actually pretty darn decent. You arent there to really cure during battle, but you can. The main benefits of this are that you can cure during downtime, saving the whm MP. A whm sub will get divine seal and curaga which is an incredible way to heal everyone after a very tough battle. It's 64 mp the whm now doesnt have to regen.
You can also help debuff if need be, or cure status ailments. If anyone here has fought a sabre tooth tiger in Battalia Downs, you will know they almost always cast a group paralyze spell. A whm is the only class that can cure this, but if you're a bard/whm you can help the whm alot by casting paralyna (costs 16mp).
Dont forget you also get auto regen at 50Brd/25Whm and increased defense vs magic as well. Also, whm adds charisma (a bards primary stat) at level ups too.
All in all, if you want to be total support and decrease downtime drastically pick a whm sub. When you hit 25 as a bard you get the song Mages Ballad. That an MP regen song. You will never have to rest again as you have hp, mp regen songs as well auto regen for hp for the 25whm sub as well.
Other subjobs work pretty decently as well. Red Mage for example. Their skill fast cast makes song casting times shorter. Plus, you get some more debuffs and a better boost to your hp at level ups. Thief is another excellent sub as well. Since Bards are best with daggers you can back stab for good damage (not as good as a main theif though) plus treasure hunteer and gil finder speak for themselves.