The main reason they have these guides is to allow the door and body to flex with out actually binding up in the whole, you must realise that a convertible has much more flex through the body than a hardtop as they do not have a turret for the extra support. A convertible does have an extra thick folded piece of steel in side of the rocker panels but it still needs these guides to help with the flexing problem, because they are spring loaded, they can adjust with the flex of the body. Main reason would be to stop paint chips when panels meet, and a door can pop open under pressure if the body is twisted hard enough. extreme cases or in an accident.