It's not really frowned upon, it's just annoying to deal with and the "community" are a bunch of cry babies that will look for any excuse as to why they keep dying other than the fact that they suck and keep sprinting back to same place where they died. The only time it gets frustrating is when an entire team are doing it all of them sat at different choke points locking the map down so all you can do is bend over and take it or leave.
I had someone doing it earlier on Terminal with Sitrep Pro and a silenced ACR and I managed to keep killing him out of his hiding spots only to receive hate mail calling me not very nice things and telling me to stop using broken weapons like the MP7, a quick look at his stats and his most used weapon? MP7 followed very closely by the ACR
Best thing to do is just ignore them or set your settings to friends only (if on 360) because all they do is talk a load of crap. I don't think I can go 3 games without getting some sort of message complaining about something or trying to insult me in some way.