Calling it a shortcut is kinda off. Skipping primer works fine if your walls are already in good shape. The real problem is people not cleaning or sanding first. I've done it in my last place with no issues, paint stayed put for years. It's about the prep work, not just the primer step.
Ever see someone paint over wallpaper glue residue? My neighbor did that, skipped primer to save an afternoon. Now his fancy accent wall is bubbling up like bad pancake batter. He's blaming the humidity, but we all know the real deal. That kind of mess is why I never rush the boring steps, even if it sucks. Shortcuts usually come back to bite you.
Seriously? Skipping primer is a bad move. Primer helps the paint bond to the wall. Even clean walls need that base. I get that prep is important, but primer is part of prep. Without it, you're risking peeling paint later. Just not worth the shortcut.