You're not wrong on your thoughts. It's the Purpose, AIO, 1-step, speed, production, etc.
However, I've seen on the run that the protection of an AIO is not as good to stand for long (please, there may be many exceptions and products that'll last equal or better than many LSP that I haven't used yet).
Talking about Meguiar's D301 Finishing Wax, in example,
It cleans the surface, and is designed to remove MF cutting Haze (for that it has to be formulated with fine abrasives, at least) and leaves a protection behind. For lesser defects, it can be used alone instead of D300 cutting compound. D301 Finishing Wax is an AIO, can be used as 1-step (I've used it many times, including today).
However, first time I've used it I regret when customer came back ~2 months after for a 'wash'... Ok, 2 months after is somewhat neglected, but ANY trace of protection was in vehicle after that period.
I had to reapply it (really fast and for free, and not talk about it to customer) to maintain my reputation.
That said, I generally do not rely ONLY on AIO for protection, I like AIOs because they polish a surface very finely, but they clean surface deeply. That's also why I believe many AIO are also called Cleaner Waxes or Sealants.
I use the AIO concept more as Fine Polish and Deep Clean than for the protection part.
My routine is actually using PBL coating booster on top of AIO. It takes ~5 minutes to do an entire car, while applying a 'dedicated LSP' can take hours (considering cure time).
That said, it's different from polishing and waxing, AIO gives you a different feel than a conventional polishing step, you're ~FREE to cover larger areas, do less passes, and it'll almost always improve the finish from where you started just by touching the surface. It'll make things glossy!, you want it's filling capacities (when polishing, I avoid fillers to the maximum).
Then, using coating booster in total you have 1 + 1/5 step, not 2 steps like polishing and waxing... Sure customer pays and receives what paid for.
That said, this is more towards 1 step than 2!
Regarding what BobbyG said above:
'Meguiar's D151 has the ability to correct, fills slightly, and leaves behind some level of protection...."
Some level of protection, from my understanding, is not 'A Very Durable Protection'. See my point?
It's the same feel I have when using others AIO.
Depending on what kind of protection I want to deliver, I'd top an AIO, even knowing it adds protection and I'm somewhat killing the purpose.
My customers (don't know how is it on US) wants to come back for maintenance wash (I mean, PAY for maintenance wash), not for maintenance AIO every wash.
So adding a top layer of protection gives me more ammo to deal with customers willings.
Hope that helps,
Kind Regards.