Yeah, watch out the 4e bashing. That can get your post deleted. (Speaking from experience.)
Anyways, Hydro, you're making me want to smash my head with my keyboard repeatedly until clear liquid leaks out of my ears.
You don't have to be better than or as good as the "pure" classes at their own job to be viable.
The point is that the Bard can serve as either a backup or a case-by-case stand-in for another character. It's helpful for the player that shows up to the game a lot to play a Bard, because if one of the other players isn't there, he can fill in without too much trouble.
And, of course, the Bard does knowledges better than anyone. My wife is playing a Jester with Perform (comedy) in my next campaign and I am both dreading and loving it because her concept is great and the whole gather information/figuring crap out won't be bogged down at all. It will be very easy.
Also, some people like being able to do several things. These people might also make strange multiclassing choices that have no bearing whatsoever on optimization. It's nice for them to have an actual balanced class they can play that at least some of the optimizers won't scoff at.
So there are several reasons to play a Bard.
One of them is not "The Bard is as Good as if Not Better Than Another Pure Class at Their Niche Role". Nobody's arguing that.
In general...
The Bard IS better at healing than a Fighter.
The Bard IS better at buffing than a Rogue.
The Bard IS better at melee combat than a Wizard.
The Bard IS better at illusions than a Barbarian.
The Bard IS better defended than a Sorcerer.
The Bard IS better at social interaction than a Cleric.
In general, the Bard is BETTER at most things than any other character EXCEPT that one thing that makes that character shine.
And that's why you play the Bard.