As usual in posts about this, I'll ask - what's the huge fuss with people caring about this? Who is it hurting? Regardless of any impatience they may or may not have, it's just nice to stand up after sitting for several hours. And if they're ready to go, that's less time for the people behind them to wait.
Personally I don't really care about the act of standing up but if I'm in the aisle seat its not that fun at the end of a long flight to have somebody's arse about two inches from your face whilst waiting for disembarking to start.
In what was is it a sign of "self-importance"? It's just standing up. Unless they're pushing their way past people or getting otherwise aggressive about it, it means nothing other than they would rather stand. Hell, they could be doing it so as to not hold people up deboarding after them.