yes, running a motherboard with an onboard gpu means it will consume the same power if you use it for a display: There are, however, two problems with this: a) why would you have bought a board with that to start with, and b) why would you be displaying something on it? I have six rigs with multiple cards, with uptimes measured in months, and see no appreciable difference in stats on primary/secondary/etc cards.

OTOH, 80+ efficiency is ignored too often, power supply efficiency is huge!