Is a teleconverter useful?

I just read this article and it got me thinking:
http://www.slrgear.com/articles/canon_tc/canon_tc.htm

If it is true that the TC is just "cropping" what the lens sees and that the best a 1.4x TC could do is give 10/14ths of the original resolution of the lens, minus any degredation from optical imperfections (seems logical); then what is the point? Would it not be better to simply crop the image from the unconverted lens. I make the assumption that the camera is sufficiently modern that its resolution is not the limiting factor.
Neil