If you are properly waited you will be neutral with 500 psi in your tank, empty BC and minimal air in drysuit if diving one. Well you have about a 6 pound swing from full to empty tank. Which means at the start of the dive you are adding 6 pounds of positive bouyancy to compensate.
Your saying add that 6 pounds to your drysuit. Yes you can do, I've played with it the air distribution in the drysuit is over a much larger area then in your BC so more drag=more effort, higher sac and possible CO2 buildup. Plus feeling like the Michelin man is rather uncomfortable.
Drysuit is for comfort, bc for bouyancy at the end of the dive the only way you shouldn't have air in your BC is if your tank is at 500 and you have just enough air in suit for comfort, which at that point you can say your suit is used for bouyancy.
I don't reach that point because I'm never still underwater with 500 psi unless the tank is going for hydro/vis and I'm sucking it down looking at fish in 4 feet of water.