I thought about this, and I'm less full of vinegar. Here's the problem:

If someone puts forth ideas, their ideas can stand on their own merits. If someone puts forth a classification system to discuss all similar ideas, the individual ideas cease to matter - it is the characteristics those ideas share in common that becomes the subject of discussion.

I can't recall a single good idea Nietschze had. But I read 'em. Derrida? Derrida I've only seen talked about and what people say annoys me. So by slamming Derrida under the label of "poststructuralism" I'm given a handy way to evaluate Derrida without having to read Derrida, and I don't think that does anyone any good.

If you're a philosopher, and you want to talk about all the different people who have said similar things, a label like "poststructuralism" is useful in shop talk. But if you want to hear ideas, it isn't.

After all, "convertibles" covers everything from the Geo Metro to the Ferrari California. One of those I'd like to drive. The other? Not so much.