IMHO the right brake should control the rear end only. I can think of two instances when I would not like to apply both, front and rear, at the same time. When making a tight turn and when a flat or other problem develops in the front.

One team that we ride with has a rear disc controlled by the captain and a rear rim brake controlled by the stoker. The stoker uses the brake only sporadically during very steep descents and only to give the disc brake a brake, so to speak.

Most teams that use a disc rear brake, us included, do not use a rear rim brake