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 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).