Depends on how your functions are written, I guess. They're not doing anything magical; you could easily replicate the code yourself. However, to write math code that is both fast and correct is a colossal pain and takes some care. I've corrected core math routines in a variety of code bases before because they had subtle or not so subtle mistakes in functions that were not properly tested.
D3DX is guaranteed to be correct, and reasonably fast. That counts for a lot.
P.S. A lot of the D3DX functions are coded in the header, if you look in the right places.