When a role is not actively being used by a tentacle, it doesn't show up on the role's field autocomplete anymore. The role remains on the deployment processes' steps though. Just because the role is not being used at the moment, doesn't necessarily mean that it wont be used anymore:

The user just deleted a tentacle with a long role name that is being used on several deployment processes' steps. Couple of min later the user provisions another machine that is supposed to use the same role. This role wont show up on the autocomplete anymore, giving the user the chance to screw up when trying to assign the exact same role to the new machine.

Perhaps it'll be good to keep the roles on the DB (and available for autocomplete), as long as it is being used on at least 1 machine, 1 deployment step or 1 variable. Open for discussion