I've read through old threads and it sounds like you guys often recommend priming with a coat of the room paint instead of primer. Is that because it's better? Or just as good, and so you only have to buy one product?

We've done some drywall patching around our house, in probably 6 different rooms with different wall colors. It seems simplest to go around with a roller and brush and prime all the repaired areas in one fell swoop, then go back room by room with the appropriate color. But is there a reason to instead use the room paint as primer?

Just trying to understand the chemistry here.