If you use Xtol as stock solution, you have the option to use the stock as replenisher as well, and keep the stock strength goodness going almost forever.
With D-76, this is a little more complicated, because Kodak dropped D-76R replenisher years ago. The formula is published, however, and ingredients aren't hard to come by. You might also be able to make replenisher from the D-76 powder -- Ilford has published a method of mixing their ID-11 with only part of one bag to make replenisher; it's possible the same would work for D-76.
The main advantage, for me, of using stock Xtol with replenishment is I get the finest possible grain and still excellent sharpness and speed -- and never have to worry about the water quality (I mix the stock solution with deinoized water, and never have to dilute). My development doesn't change with the seasons, as can be the case if you use well water to dilute developer (pH of the water can change). The same is true of D-76.
Of course, with free D-76, you could just develop with the stock solution and add time as you use the capacity, and then discard it after something like 8-12 rolls per liter.