A Bard buffs and heals some. I don't think a Bard works a sole healer (let me know how if you manage) because when your tank is low on health, buffing him sure as hell isn't going to save him and he has a sodding cooldown on his heal. Using a Cleric and a Bard means losing 1 offense, and a Cleric alone would be more than sufficient.
What's the point?
i am incapable of coming up with witty signatures so this will do.
It's probably true that the Bard is not a good healer at lower levels (skills not available, and limited items).
Later on things are a little different.
With 2x Book of Jabe (-12 sec on cool downs) your Song of Vigor is practical always up and with Healing Cords makes it even better. This alone keeps everyone topped up. Finally Echo of Renewal turns your single heal into a group heal.
What I personally find is that the Cleric and Bard can be interchangeable, making party combos always interesting.
For reference my Bard spec:
2x book of Jabe lvl 5: Song of Vigor lvl 10: Stoicism lvl 15: Song of War lvl 20: Healing Cords lvl 30: Echo of Renewal
Bard is more of a support healer, offering unique attack buffs the cleric doesn't offer. He's pretty sufficient as a main healer (given your team has some life-steal gear equipped), so you don't need a cleric healing too.
Putting a bard w/ 2x -6 sec cooldown trinkets on a dps pretty much makes up for losing an actual dps character since you're able to keep song of war / vigor up constantly. I pretty much never have to take my cleric off my knight this way also. Bard is a beast.
I can easily heal my whole group with my bard alone... It gets really hard when i reach the 80th enemy in the third arena... because the archers just do too much damage if you don't pay close attention to cc and the healthpoints...
But with two Books of Jabe and the Song up all the time + op group heal every few seconds a bard can heal really good...
The bard wasn't attractive to me until he hit 30. Then you can toss two Jabe's on him, and the buffs end up being more useful than the pure healing of the cleric. The one exception is if you're using the Knight revenge strategy. In that case the Cleric's armor boost is much more necessary to help ease all the incoming damage.
@Tso: I assume upgrading the bard's weapon will improve his heals and buffs. It's difficult to measurably check this though, so it's just an assumption.