This is a really fascinating post. :D You're exactly right about the purity and canon issues, for me. I think most of what can be brought to light about any given character can be brought about using either OCs, or characters in the canon universe. I cannot think of a single situation where a character from another show would be absolutely necessary. And since for me, it's usually all about character, less so than scenario and plot, I can't see a reason for it. I want to see how characters are changed by those in their orbit, not those forced into their orbit by the writer, if that makes sense.

I confess that I almost never read crossovers, no matter who writes them or what the fandom is. This is my sad face *points*, because I'm sure I'm missing a lot of cool stuff, but. I have this thing about crossing universes that don't go together, and I think most universes don't go together. It probably has to do with being a complete canon whore. Also, the chances I'll like it are substantially decreased if I don't watch or like the other universe in the crossover, because I read fan fiction to see additional character revelation, and if I don't care about the characters, there's no incentive to read. Stories set in space/stories not, or stories set in two different time periods or universes, are a definite no-go.

Interestingly, there are a few universes I can see crossing effortlessly. One of them is Supernatural with Miracles or Angel. I think it has to do with similar themes about evil and similar types of monsters and demon-fighting, etc. It's plausible that these characters would cross paths in pursuit of common goals. Also it has to do with concurrent timelines; these shows are recent, and there's not a large gap between their timelines. But for the most part, as soon as I see 'crossover with' in the notes of a story, I click right on by.