My kydex plate is doubles only with one exception, and my SS plate will go back and forth which requires adjustment, but I dive singles almost never. It is more a loaner option than anything. The one exception for singles in Kydex was during my instructor skills demos where I had to use a weight belt for stupid gear drills that required removing my rig and staying stable in 8ft of water. Not a normal scenario.... Other than that the Kydex is for wetsuit steel doubles diving when I'm teaching. SS for drysuit doubles diving, and all singles diving since I choose to use AL80's instead of steels if I'm diving in warm water.
I'm weird though, so write that statistic out of the normal realm
@stuartv yes it is, but it also mitigates the risk for pinch flats, and when diving off of the coast of NC, makes tank switches a helluva lot easier. Not something I would do for travel and I think I said that, but if I was regularly diving singles in the mid Atlantic, I'd be using STA's. I don't since I dive doubles, but have plenty of buddies that do. Roughly $60/tank and since you are normally doing 2 tank dives, it's well worth it for the convenience of not having to stabilize the tank, lift the whole rig off, get it stable, try to align the cam bands in the right spot on a pitching boat, hold the rig and pull the cam bands, etc etc. All of that vs 2 quick spin nuts, pull the plate off and lay it down, swap the wing over to the tank that is still strapped in and stable, put the plate on the bolt holes and spin it back on. It's much faster and much easier.
It's a convenience that all of my buddies that still dive singles are willing to pay for. It's a different way of doing things and for travel, the DSS setup is infinitely better overall, but like everything, there are compromises to be made. Pros and cons and you have to base that on what you're doing and make sure the style of diving fits the rig you buy. For me? I'll take my double 120's and not have to swap anything over, but they don't travel particularly well so I have my DSS rig for that.
I'm pretty much in the same boat. A kydex plate for warm water and double steels. A SS plate for cold water and doubles, and another (currently SS) plate for single tank (which would always be warm water, for me).
And I have to say, in the last almost year and a half, I have ONLY dived my double 120s when diving in NC. For me, it's not about anything to do with changing tanks between dives. It's all about having redundant gas and regs. It just feels safer. Especially when diving with either an insta-buddy or a buddy who is going to be focusing on taking photos while I'm doing the same.
But, for single tank diving where a tank change is required between dives, on a pitching boat, having the string that sets the plate height on the tank and having QR straps makes a tank swap super easy and super quick. Way easier and quicker than when I was using a DSS plate, wing, and regular cam bands. Unscrew the (DIN) reg and let it hang/rest on top of the wing, pop the two QR buckles and the whole rig is just hanging on the tank. Lift it all up by the string and shoulder straps, put the string around the neck of the new tank and let the whole rig hang. Do up the QR buckles at my leisure, with no futzing about getting the rig set at just the right height on the tank. Dealing with the reg is the same either way. I think I could pop the buckles, move the rig, and re-do the buckles on a new tank faster than I could undo two Delrin thumbwheels, move the rig, and put the thumbwheels on the new STA. But, really, I imagine it's close enough be a wash, timewise.
Now that I think of it, though. when you swap to a new tank that already has an STA on it, how do you manage the two bolts that go through the STA? Do you move the bolts from one STA to the other? Or does each STA have its own bolts? It seems like if you move the bolts then especially moving the bottom one would add time. And getting it to stay in place when you try to push the wing and BP on seems like it would be tricky? Don't you have to be really careful or otherwise you end up pushing the bolt back out of the STA hole? If each STA has its own bolts, what keeps them in place? Does each STA have its own wingnuts or thumbwheels to hold the bolts when there is no BP mounted?