True. I guess the other proviso is, I wouldn't use a TC on a mediocre lens - would want to restrict them to good quality glass, so image degradation is less likely to become a problem. If you do that, the apeture issue tends to be covered anyway.
I've got a 2X TC in the Kenko Pro 300 range, it's been really good. So far I've used it with 200/2.8, 100/2.8 Macro, 300/4 and 80-200 2.8, and it's been great with all of them. I chose it because it works with those 4 lenses.
I don't recall if it's compatible with the kit lenses, but frankly I think it'd be a bit of a waste of time anyway