Yea this part could need an update. Uppon joining the internal value is used, as with a normal select box as well.
The admin side internal value is restricted to no special chars or spaces in an attempts to auto secure the select box on the frontend. Personally would like to keep it at that way, but indeed makes it hard to use the display text later for the member.
Easy fix would be to just use 1 box for the admin, and uppon joining maybe validate in a similar fashion as with the categories
We think of something to make it it easier, preferable without removing the second box, as that might really fu** some sites using it already.
Also involves quite some texting as its used by checkboxes and radio buttons as well. So iam not sure anything happens for that for 1.0
Properbly stick with what Mark posted for the time being