Ah, but that's assuming that the person is that sensible in the first place.

*he can think of one or two less sensible types he's met...*

But in any case, that's certainly true as well. Not to mention, the longer you wait, the greater the gap in their training. Some gaps eventually are unavoidable, of course, but if you have one pokemon that's much stronger than the rest, it can be tempting to solely rely on that one.