I use it here because it is a short sensible abbreviation. "Manchester United" is just too long. I don't like "United" because there are lots of other Uniteds and "Man United" seems like unnecessary typing.
Saying you wouldn't hear it at "OT" is not really the point. I'm sure you wouldn't hear "OT" at Old Trafford either but you wrote "OT" presumably because it is a simple short abbreviation. And at Old Trafford if you say "United" there is unlikely to be any ambiguity about it.
I think it looks pedantic to complain about it on the internet, particularly when people deliberately change the name of other clubs in very derogatory ways in a vain attempt to be funny.