Pellicle mirrors offer some of the advantages of a rangefinder camera in an SLR. When taking a picture with a typical SLR camera, the mirror must move out of the way to allow light to pass from the lens to the film plane (or digital image sensor). Moving the mirror makes noise, introduces vibration, and causes the viewfinder to "black out". The camera also must have a slight delay between when the shutter release button is pressed and when the shutter actually opens to allow time for the mirror to move. Quiet operation, low vibration, absence of blackout, and fast response are features usually associated with rangefinder cameras, as they have no mirror at all between the lens and the film.
A pellicle mirror, such as the one in the EOS RT, always reflects some of the light up to the viewfinder, and allows the rest to pass to the film plane. Therefore the mirror does not need to move when a picture is taken. Having an uninterrupted view through the finder is especially helpful when trying to pan along with moving objects, or during sports photography. Although the viewfinder does not blackout, it can however go very dim as the lens stops down to the taking aperture, so this advantage is greater at wider apertures.
A disadvantage of a pellicle mirror is that the light diverted to the viewfinder means less light is available to expose the film — in the case of the EOS RT, the lens effectively loses 2/3 of an f-stop. A less reflective mirror might be used, but then the viewfinder would be darker and harder to use. The advantages of a stationary mirror are less important now that modern cameras can move the mirror back and forth very quickly, reducing the shutter release lag and increasing the amount of time the viewfinder is usable. Cameras such as the Canon EOS-1D and Canon EOS-1D Mark II are able to take 8+ frames per second while moving the mirror back and forth, which is very close to the 10 frame per second rate of the pellicle mirror-equipped 1N RS. The EOS-1v with PB-E2 power booster (2000 release) and the recent EOS-1D Mark III are capable of sustaining 10 frames per second burst rates without the use of a pellicle mirror.