20a outlets are also expensive compared to 15A ones. When redoing the electrical in the house I inherited I put a dedicated circuit in the living room for the TV/game consoles/etc since my daughter's gaming rig would be going in the living room as well; since I hadn't decided to put the recessed panel for the TV wiring in at the time it was the only outlet on the circuit and code dictates that a 20A circuit with a single outlet must use a 20A receptacle.
I can buy a 10-pack of nice 15A Leviton Decora Edge outlets (that use lever lock connectors instead of screw terminals) for $26 ($2.60 each), but basic 20A tamper resistant outlets (which newer editions of the NEC require anywhere a small child can access them) are $6+ a pop.
When nothing outside of my electrical room (where the servers live) has need of a 20A receptacle it's kind of pointless to spend the extra money on them, but the extra couple bucks on 12 gauge copper is always wise.
It has long been my understanding that a single regular, bog-standard 15A duplex (ie, dual outlet) receptacle meets the multi-receptacle requirement of a 20-amp branch circuit.
If my understanding is correct, then you overkilled it and you could have saved a few dollars, at least one welli-intentioned rant, and still have been compliant with NEC.
A literally-single 15A outlet like a Leviton 16251-W would not pass muster, while one dual-outlet example of the 15-amp lever-lock devices you mention would.
Well, the rant still applies either way - the 20A outlets are 2x the cost, that's the big reason why the aren't routinely installed. I was thinking when I embarked on the project "oh, I'll just put 20A ones everywhere because why not?" and immediately decided not to when I looked at the prices for the packs of outlets....
I had the luxury (or curse, depending) once of owning a home that needed all of the wiring replaced.
Being the kind of person that I am, I overbuilt things as I felt was appropriate. As part of that, I certainly wanted to install 20 amp outlets (even though I've never held in my hand a 20 amp plug).
The cost of that, vs good spec-grade 15A duplex outlets, was insane.
I know that the only difference is using one T-shaped contact instead of a straight and some different molds for the plastics. The line producing T-shaped contacts already exists, and so do the molds. Every 15A outlet sold today can transfer 20A safely.
It should be pennies difference in cost, and it was instead whole dollars.
Sucks.
(I'm reasonably certain that we are going to be broadly stuck with this low-current, low-voltage business until something very different comes along, and that any of this is unlikely to change in my lifetime.)
I think this is mainly because the 20 amp outlets are kind of ugly, and the fact that barely anything actually uses a 20 amp plug.
In my house, almost every circuit has a 20 amp breaker and 12 ga (yellow) romex, but only a couple of outlets are 5-20.