I'd say if there is going to be a rule, it can't be subjective.

For example, a one-wheeled 720 or NST using spinning forces to manoeuvre could be considered mobile at one event, but immobilised at another event with a different judging team. That just leads to inconsistency and complaints among roboteers. The rule needs to be clearly defined so that there is no grey area that could lead to disputes, otherwise there's no point in adding a rule in. Not that we need one anyway