At sizes above 10mm, I don't think a 12-point socket is more likely to damage a nut than a 6-point. Because of the slightly loose fit that any socket has on a nut by design, 6- and 12-pointers end up contacting the nut pretty much the same way.
I wouldn't be surprised if impact sockets are 6-point not so much for the sake of being kinder to the nut, but rather to make the socket itself stronger.
And to state the obvious: a 6-point socket won't cut it for square nuts or 12-point nuts (which I've encountered exactly once in my travels--connecting rod cap nuts in a 1960s GM V8).