You only really need masking blades if you are using a long lens, and the shade isn't long enough, or if you are using fairly extreme movements and the shade isn't flexible enough to deal with it. For instance, if you have maximum front rise, and you have to set the compendium shade wider than normal for that lens to avoid vignetting, and then you want to mask off the other three sides of the image. Alternately, if you are shooting 6x12, you might want to use the masks to conform to the shape of the frame.

The goal of the compendium isn't only to prevent bright light from falling on the lens, but to minimize non-image light that can reduce contrast by bouncing around inside the lens and reflecting off the inside of the bellows.