Hi...new drywall will absorb the paint like you wouldn't believe. I have also just gotten through drywalling a basement and we had put 2 coats of primer to seal it up before painting. The reason is that the bare paper on the drywall will soak up the paint/primer and will need more coats to get a good finish. The paint will go on better and the primer will hide any stains, slight surface imperefctions, and things of that sort. I recommend a good primer under the paint, and if this is not done you will need to put at least 3-4 coats of paint to get a nice finish. It is not necessary if you want to paint a bunch of coats on, but it still will give you a better end result.