Rollei sheet film holders are pretty but I'm not sure if they are of any practical use. Apart from the Zone System, I can't find a good reason why anyone would like to torture himself with using sheet film smaller than 4x5.

In my experience it's not the screen that is dim in some Rolleicords/-flexes, but the mirror that has partly desilvered. The stock screens present in later Rolleis (at least from the 'Cord V) come with a Fresnel and they are quite bright, but what is more important: contrasty.