But you may find as I did at a freind's. You never know what you're gonna find in some older homes. A single 15A breaker was powering every outlet in his kitchen but the firidge, with No CGFI anywhere. I left the 15A out's but found the first in chain and gave it 15A cgfi. Then added a mic out for him too on an added 20 in his box. Point being that the ten outs all over the kitchen wouldn't use the whole circuit to breaker for trip before I could talk his cheap A into putting the proper 20A outs into his walls and breaker in his box. At least it was properly wired with 12ga! You really just don't know for certain what's really going on vs. what's right in some houses that have had handyman owners doing work on them til you get into it.
I think what 101 was trying to say is that it would be pointless to use the expensive 20A outs on a 15A circuit, as well as potentially dangerous to give someone the impression of a heavier circuit to plug a dozen appliances into. Were it to have a lower CGFI up-line it would be easily found out but if it had 14ga hidden in the walls, that could be a bigger issue when excess draw turned to heat in an enclosed space without tripping the breaker or even having a 20A breaker in the box without proper outlets downline, which may burn a single outlet or the house.