A bard can slow, and I can buff with other classes before I leave. This makes the cleric my number one choice.
If you don't anticipate having access to lots of buffbots, a druid is very nice due to the DS++ dps they have, as well as extra dps on the side. This is assuming you're not tanking super tough mobs, but relatively weak ones that won't tax healing abilities too much, and for whom the DS dps will be more substantial. You'll plough through much more content faster at this rate, and this is what Felyn did to farm pretty much his entire tome set and cash for his charm. I'm pretty sure that if he were put back in that same position, he'd choose the same setup again.
A shaman's slows can significantly make up for the lack of healing power, and the extra dps and buffage (again, if you don't anticipate having an army of buffbots) can be substantial. True, a bard can slow as well; but it does mean the bard won't be doing as much dps,as you can only get two detrimental pulsing songs on at a time. A shaman wouldn't be my first choice, but there's definitely some advantages here.
Aegolism is the only buff the cleric brings to the table, but it's big enough to really stand out. The additional healing power helps a lot on weak tanks like bards particularly if you are fighting tough hitters, and if you are fighting undead or pulling lots of mobs at a time, clerics can do some additional dps.
I'll be the first to admit that I have a cleric alt, but I modeled my duo more after the bard who came before me and less by this kind of careful analysis. More than once I've wished I'd have made a druid instead, but I'm happy with Avaline anyhow.
Long story short: it's not really as black and white as it appears. If you're the #1 - #5 bard on the server with the gear to tank low end raid mobs for cash drops and you have access to any buff you like, there's little reason to take the classes with less healing ability. (Of course, if some of the proposed enchanter changes go in, that might change!) If, instead, you're looking for a good duo partner to get you up to that level; or you are looking for a duo partner which will be good for farming exp and cash from ~ lvl 55- 65 w/3 tomes, the druid and shaman really bring extra utility (including much needed dps) to the table.
Hope that helps!