It would be if the length of a tag's name wasn't as limited. There isn't enough room to describe the usage of a tag if the usage pattern is strict, just the nature of it. (And then, who wants to read the usage pattern every time the tag comes up?) It's a useful note to the author or authors.

I hadn't considered the encouragement of junk tags. I've seen that go badly elsewhere, but kate_nepveu seems reasonable in stating that people who use tags for fun or conversation will do it anyway, and people who don't won't. I don't know of any study to back this, but it would be enlightening to see.