Alan, I thought the preservative action of Sodium Bisulfite in most "part A" solutions was simply due to its acidity, which lowers the pH of the solution and therefore slows or inhibits oxidation of the developing agents (most developing agents, but especially ones like Pyrogallol which oxidize quickly in alkaline solutions). As I understand it the elegance of using Sodium Bisulfite for this purpose is it then converts to Sodium Sulfite when the alkali (usually Carbonate but Metaborate in the case of PMK) part B is added, which gives the working solution the required/desired amount of Sulfite.
I wonder though if that mechanism would work with Borax or if the alkali needs to be stronger.
On balance Gerald is probably right. Separating the alkali seems to be far more necessary in the case of acutance and/or Pyro developers where there is relatively little preservative, a low concentration of developing agent(s), and a relatively high pH. D-76 has none of these properties.