Don't we? Don't you have to apply for an Ubuntu membership, telling about how you've contributed?
See details here about Ubuntu membership. (This questionThis question might also help, as well as this meta question.)
I think users who have contributed extensively on Ask Ubuntu, even if they aren't significantly active anywhere else in the Ubuntu community, surely would be eligible for Ubuntu membership.
However, I should add that usually membership is granted based on contribution in multiple areas (see Marco Ceppi's meta answer explaining this).