One of the big problems with not compiling your own kernel is that most kernels will run basic i686 or i586 code on your machine.

In RedHat 7.1 my Athlon Thunderbird 1.2GHz is detected as only an i586 processor. I can recompile the kernel for the athlon directly, which will incredibly optimize it, so that my linux box isn't running as if it were simply a Pentium 1.2GHz (a slight exaggeration, I know).