@ John Irvine - the results I have noticed with Xtol is a slightly finer grain, a sharper negative, and a tonality that is not as 'sterile' as with completely new developer. In the shadows I see similar results as with fresh developer, the midtones seem better separated, and the highlights take on a quality that resembles more of something like Rodinal; the highlights seem 'quieter' somehow, calm. Fresh developer, in my opinion, tends to put too much punch in the highlights, making the tonal shifts more abrupt. For some subject matter that may be desirable. But for my work, I prefer the look of replenished developer.
I hope that aids somewhat in explaining the advantages I see to replenishing. Economic reasons aside.