Here's my thinking, and let me know if I'm way off: if I use it to prime the walls, even though the mildewcide aspect will be wasted if there's ever a mildew problem with the RL (unlikely given the ventilation), the seal that the Zinsser forms behind it should protect the drywall/compound better than a normal primer would.
Well, lemme explain Slickshift's point first: He's saying to use a regular primer because when a company makes a dedicated primer, then they don't have to make compromises. The primer will use a binder resin that sticks best to the substrates it's intended for (like drywall and joint compound as opposed to a surface that's already painted) and the primer will contain huge rocks that are almost large enough to see with the naked eye called "extender pigments". These rocks plug up the porous surfaces of bare drywall and joint compound so that most of the primer remains on top rather than be absorbed into the joint compound and drywall. With a thicker film of primer over those porous substrates, then none of the paint is absorbed and you end up with uniform, thick coat of paint that hides better.
If you can buy Satin and Semi-gloss Zinsser's bathroom paint, then it can't have very much extender pigment in it. That's cuz it's the amount and the fineness of grind of the extender pigments in a paint that determine what gloss it dries to. A flat paint or primer will have lots of coarsely ground extender pigments. A satin or semigloss will have much less extender pigment, and what it has will be very finely ground. So, to plug up a porous surface you need rocks in the paint a bit bigger than the holes you want to plug, and plenty of them so that the least amount of paint gets absorbed before the holes get plugged. Zinsser's paint primes by plugging up the holes in the substrate with paint. You just let that paint dry completely, and add another coat of paint. It's not the best way to do it, but after that first coat, very little paint will be absorbed into the drywall or joint compound, almost all of it'll dry on top of that first coat. And, the third coat won't get absorbed at all.
Also, by their very nature, latex paints are more permeable to humidity than alkyd paints. So, what Slickshift is saying is that using the Zinsser's paint as it's own primer isn't a good idea cuz it won't perform the job of priming as well as a dedicated primer cuz it won't stick as well nor plug up the porosity as well as a dedicated latex primer.
My personal view is that, maybe true it won't stick as good, but how hard does it need to adhere to provide good service? No one's going to be pulling on the paint or scraping at it. Suffice it to say that you won't be the first or last person to use a paint as it's own primer, and people who do that aren't warning others not to, so they're happy with the results. I think you'd never know the difference AFTER two coats of Zinsser's whether you primed the walls with the Zinsser's paint, the BM primer Slickshift suggested, an alkyd primer or your garden variety general purpose drywall primer.
Slickshift is also saying that any latex product isn't going to form a very effective moisture barrier, and the result is that humidity from the bathroom could potentially pass through the the drywall walls and into the rest of the house. There is truth in that; latex primers and paints will allow H2O molecules to pass through them. But, you have to have sufficient humidity getting into the walls to cause problems like mold and mildew growth. Humidity will get into a wall through electrical outlets, wall switches, etc. It'll get into your bathroom walls if you have a recessed mirror or even through your ceiling fan. However, those small amounts of humidity getting into walls and attics don't cause any problems. It's when you have a water leak, or water getting in some place where it can't get out that you have problems. There's latex primers and paints on gazillions of bathroom walls, and most of them will never have a moisture problem.
I know that when I've had paint peeling on a bathroom ceiling (cuz I used a cheap paint), scraping it down, skin coating it with joint compound, sanding smooth and priming with a latex primer and top coating with Zinsser's bathroom paint was sufficient to prevent the peeling problem from recurring. I know the peeling problem was due to inexpensive paint softening up and loosing it's adhesion under humid conditions. It seems to me that if covering that inexpensive paint with Bullseye 123 (a latex primer) and Zinsser's Bathroom paint (a latex paint) was sufficient to prevent the problem from recurring, then those two latex films formed enough of a moisture barrier to prevent peeling of the inexpensive paint they were applied over.
I do disagree with Slickshift's contention that bathrooms are the most heinous environment for coatings. I have 21 bathrooms in my building, and I WAS having paint problems in all of them until I learned a bit about paint and found out that the problems revolved entirely around the fact that I was using inexpensive PVA paints and true oil based paints in them. I wasn't using paints intended for bathrooms. So, I think Slickshift's opinion may be affected by having seen too many bathrooms painted by homeowners who cheaped out on the paint they used in their bathrooms.
Here, if you want the ULTIMATE painting strategy:
1. Prime the walls with an INTERIOR ALKYD primer.
2. Paint over that with two coats of Zinsser's PermaWhite Bathroom paint on the walls and apply a third coat to the ceiling and top 12 to 24 inches of the walls to get the same mildewcide reserve there as if you'd use the Zinsser as a primer!
3. Now apply your metallic RL where you want it, as long as you don't cover the top 12 to 24 inches of the walls or any of the ceiling.
The interior alkyd will plug porosity, stick very well and form a film that's impermeable to humidity. The Zinsser's will kill any mildew IF it grows through the RL paint before it can start growing on the alkyd primer or into the drywall or joint compound. And, the RL paint will impress your friends and neighbors that you could afford Ralph Lauren paint and still eat meat on Sundays.
Not sure if I answered your question. I think you'll get good results that will last a very long time whichever primer you use, but I do agree that Slickshift's point that a dedicated primer will do the job of priming better than Zinsser's Bathroom paint will. I don't agree that it'll ever make a material difference, tho. It's not like you're going to have a problem using the Zinsser's, you just won't have the warm fuzzy feeling you get from using the best product for the job at every point in the work. But you'll never know the difference unless I told you that.
If this is a legimate use of the product my question then is, will there be any issue with trying to use the RL on top of the Zinsser?
I wouldn't anticipate having any more problems painting RL over Zinssers than you'd have painting anyone's paint over anyone else's paint. Both are still latex paints and they still form a film exactly the same way. So, if you know how latex paints form films, then you'll know that the RL SHOULD stick well to Zinsser's or any other latex paints. I would, however, use the Satin Zinsser's or give the Semigloss Zinsser's a light sanding before painting over it with the RL paint for maximum adhesion, tho. (It'd bond to the underlying Zinssers better simply because by sanding, you provide more contact area between the two paints. Same adhesion over a larger contact area per square inch of surface area amounts to "better" adhesion.)
Hope I answered your question(s). There's more shades of grey here than there are blacks and whites.