Why isn't purity & co. inferred for all functions?

[Trass3r]

It's already inferred for templated functions, so it's perfectly possible.
Also it should be checked anyway, so I get an error message if I marked  
something pure or whatever while it actually isn't.

The only functions where it can't be inferred and manual attributes are  
really needed are function definitions (i.e. no body available).

What speaks against it?