That's never bothered me, but I guess I see your point. I always figured that the point of having a 20A circuit with 15A receptacles was so that you can plug in and run two 10A devices without popping the breaker.
Stupid question: are there any common household appliances that draw 20A? I've only ever seen the T receptacles in non-household uses.
This is, of course, what I was talking about. I do appreciate the nuances of the 15/20 T-socket and the 6-20/5-20 symmetry; NEMA is super cool (aside from the fact that bladed plugs that aren't twist-lock are sort of lame/less safe in general, at least compared to the UK standard bladed plug). It's a fun rabbit hole to venture down: my occasion was having to adapt straight-blade wall box receptacles that the electrician installed to the twist-lock of the PDUs we had bought.
There's also a fun and confusing mess of competing DC connector standards. I haven't decided what 12/48VDC connectors I'm going to put into my new house build yet (beyond the obviously mandatory 5V USB).