The new primer-and-paints are fantastic. Actually, most quality paints have more or less had almost this level of coverage for years now.
The only time primer is really all that necessary in an interior setting is if:
A) it's new plaster or drywall. It needs to be sealed with a good primer coat first, then painted with one or two coats.
B) There's oil-based paint on the walls, and you're going to paint with latex. Even then, you should scuff-sand the existing paint to assist with adhesion.
C) There's a dramatic difference in existing and new colours. Say the existing walls are white and you'll be painting a dark, navy blue. Tinted primer is the way to go, otherwise you'll likely end up slapping three coats of paint on.
D) If there's nicotine, cooking stains (grease) or water damage. These conditions usually "bleed through" latex paints. They should be primed with a heavy-duty primer sealer like Kilz or Binz, etc.
I think that pretty much covers it
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