Originally posted by: seemingly random
It depends on the existing paint. If it's "old", a coat of primer will make life easier. Primer is really a sealer - mostly transparent. If it's needed but not applied, the new coat may not end up the expected color. It could also require much more paint since the drywall will absorb more.
If the existing is "fairly new", the new paint could be applied and if not satisfactory, applied again. This would be appropriate only if the new paint doesn't cost much more per gallon that the primer.
If this is fairly new construction with the original paint, consider it "old". Contractors always skimp on paint.