I agree the usability could be improved there, I've added a to do in the project tracker to look into this. I think the solution will be to either wrap an update panel around the roles so that it doesn't do a full postback, or I may make a link that opens a colorbox dialog page. Their is also a potential issue on this page for sites with lots and lots of roles, it could cause the too many form elements error that we had to solve on page settings and module settings and site settings pages after the asp.net security update set a limit on the number of form elements. Removing the role dropdown and adding a link to a dialog popup page would solve both problems and also make the page load faster since it wouldn't have to load the role list until you want to manage roles.
Best,
Joe