I'm going to throw a rock at a popular misconception (well, popular with some people at least)

I don't think you should keep a fish to fix any problem. In other words, you don't keep something to keep the bottom clean or solve your algae problem.
They're all fish with specific needs.

As to cories... I really like them, especially in groups. It's only common sense that keeps me from rushing out and buying a dozen pygmaeus or habrosus.

And the age old question of "why do people keep..." can be said about many things. You're not fond about cories. I don't see the point of all those "sharks" and other 10inch+ fish that people like to keep. Tastes differ.