I'd say allow it. Most people have the ability to customize a button on their Mouse anyway. I don't use the double click button but I see everyone else using it.
Im about 99% sure viruz has been using it for years, among some other gow/ef players. it's pretty obvious just watching games.
and yes LB.. you do need a button designated for double click, or the macro to lust that fast... if you didn't, nobody would be doing it.
Go watch 00joe's stream, mouse's stream or any of viruz' replays and tell me if you can tell they're not actually doing the "B click click B click click" sequence when they lust.