Well one of the reasons I mentioned that is some people just assume that Behr is not a "premium" paint simply because it's available at Home Depot, and that's spec'd for bare drywall.
With regards to price, it might seem primer is less expensive, but not necessarily. Let's say for sake of argument that you're painting a 15x15 master bedroom with 8 foot ceilings. You're going to need 2 gallons to do 1 coat. Let's say the primer is $20 and the paint is $40. 2 gallons of primer and 2 gallons of paint is $120. That assumes you tinted your primer and the paint goes on in 1 coat. Most people swear by 2 finish coats for correct color, which will take 3 gallons, so now your cost is $160.
Now skip the primer and just use 2 coats of paint. That's 3 gallons at $120.
So "priming" with paint is actually cheaper in this pretty typical case. Or at best the same price if you can get away with 1 finish coat.
Primer is used because it has good adhesive and sealing properties that give you a good even base to paint over. After primer is applied then also your opportunity to fully inspect the drywall for defects before applying finish. The fact a half decent primer cost less than the finish coat is just a bonus.
You can certainly use the paint as its own primer coat but I think it would bite you in the butt real fast if you were using that as a system on level 5 finishes. Same reason I wouldn't ever spec a PVA primer.