Your vocab and knowledge seems to indicate your a general contractor. I can appreciate that, but don't sell yourself off as a knowledgeable painter. The above posts regarding primer are gobbledygook, and as a man with about 25 years of day in day out experience, I find them hard to understand. Where do I find fault: Yours in red, mine in black.
You don't HAVE to prime. Priming can accomplish several things, but make sure you need to accomplish those things that your cover coat can't accomplish before you pay for primer and do the labor.
You do "have" to prime. Finish is not formulated to "penetrate", not "key", into bare drywall. It will not bond, even when rolled. Primers penetrate.
For example, if you are going to apply 2 coats of decent quality flat paint over new drywall, I would probably not bother priming. The first coat of good quality flat paint is basically your primer, and you have no sheen issues.
There generally are no sheen issues with flat to begin with.
Even with good quality eggshell paint, 2 coats right over drywall might work fine. Again, the first coat is essentially a primer.
Wrong. This is where you will have sheen issues. If the first coat of eggshell is used as a primer, the bare sheetrock will draw the sheen out of it, leaving it less than an eggshell. The second will consequently be called upon to finish the sealing job and it itself will sacrifice some sheen, leaving the final finish with a less than perfect sheen. I've learned this through my own work and from doing experimental test samples, something I do fairly regularly.
They sell primer as saving you money, but that is not necessarily the case. Let's say you're painting a 16 x 13 bedroom with 8 foot ceilings. That's about 460 sf, so you'll need 2 gallons of paint for 1 coat. You will also need 2 gallons of primer. If the primer is $20 and the paint is $40, that will cost you $120. Now if you want to put on 2 coats of just the paint, that will require 3 gallons at the same cost of $120.
This makes no sense. Since when are paint companies interested in you saving money. They sell primer because it is the fundamental element of any paint system. One prime, two finish on bare substrates has been the industry standard forever. There are exceptions, and I don't acknowledge or include the "paint and primer in one" hype.
With good quality flat paint, you will be fine. In fact, in terms of color, you'll be better off because you just put on 2 coats of exactly the same color, for the best in hiding and covering. In fact, quite frankly, you can often put a good quality flat paint right over drywall in 1 coat and have it come out fine (my experience is mostly with SW SuperPaint in this regard). The paint manufacturers do not want you to consider this, of course. Today's high quality paints are better than old ones - and you can apply them generously if you like since they are thicker. You can roll a gallon to 320 sf, for example, instead of 350-400 sf.
You're clearly not a paint expert, nor am I, in all honesty. But, to tell a noob, as you call them, that one coat of any flat over bare drywall is fine is clearly indicative of a get er done mentality, and that is frowned upon by the pros here. One coat coverage is a hype like paint/primer in one. I have never found a paint where color and sheen fully develop with one coat, even Aura, and most especially over bare drywall.
When different areas of drywall (paper, joint compound, etc) have different porosities, you usually want to correct that first. This is more an issue with putting on eggshell or satin paint with the finish coat (I count Aura and Duration as eggshell - matte does not mean flat in this case.) For these and other cases, you really need a drywall sealer (the sealing part being more important than the "priming" part, depending on how you translate the meanings on primer labels.) And again, a coat of the actual paint might be able to act as that sealer.
??????? The difference between a primer and primer sealer is less about what it does for and to the sheetrock than what it accomplishes for the finish coat. Both will seal drywall, but which is chosen is a function of the selected finish. A straight primer will, again, draw sheen from the finish, whereas a primer/sealer will not. Generally, I think it's just easier to not worry about which sections of the new drywall will or will not present a problem and just prime the whole damn surface.
Of course there are texture issues as well. This is especially true when a thin coat is sprayed on (even in flat paint). If you can see through to the texture difference between the drywall paper and the taped and mudded seams, it has not been done right and you can tell a primer (or even a base coat of finish paint) has not been used.
Again, ??????. What does this mean; you can tell a primer (or even a base coat of finish paint) has not been used.? If it hasn't been coated with primer or finish, what has it been coated with? The issues you are talking about are generally caused by not priming, the very same advice you suggest OP doesn't need to worry about. I generally don't, and can't really, short of horrendous drywall work, determine what a ceiling is going to look like until the prime and first coat of finish are applied and dry. I stick to the tried and true, by the book, method of doing projects and rarely do I ever have issues.
Using finish as a base coat is a flawed shortcut. It's been discussed here a few times, and it's been unamiously frowned upon. In spite of what paint companies promote as paint/primer in one, it's not a generally accepted practice, and as pros we can't recommend it. Ben Moore advertises Aura as such, but the tech hotline will tell you no, you have to prime. It's marketing hype. There are a lot of things I do that experience as a pro allows me but that I can't recommend others do. I don't relish going after you here, and your opinion is certainly welcome. I just wish you wouldn't approach other's advice so dismissively and assert yourself as an authority, because you're not.
Joe