I'm not sure, to be honest. There's a lot more champions that can do what he does 10x better.
Bard is still a new champion, and not a lot of people have mastered his kit.
Sure, he's got poke with q and his meep-empowered auto's, but that's all for damage...
He's got a weak heal/movement speed buff, A stun/slow, and a tunnel because why shouldn't he.
I guess it's useful for invades/escapes, as it makes the team quite slippery, if followed by a q stun when being chased.
He can't really peel that great, and he's often built full tank, meaning that he lacks all the cc a tank should have, and replaces it with a weak heal and a tunnel that can't be used in teamfights.
Compared to other supports, he's not very high tier. If you can get a good q in lane, that'd be nice, but there's no cc chains.
*Forgot about his ult.
His ult makes sieging and diving easy, as it can shut down turrets. His ult will always get at least one teammate killed, as, since it's a zhonya's for all 10 players, it will put someone at a positioning disadvantage. If you're not catching/following up your long range ult with a Righteous Glory, it's not really that good of an engage, as short range it will put your team at a disadvantage, and unlike a sejuani ult, etc. You can't damage them while cc'd.