Originally posted by PPPPPP42 A lot of the time the need for it is overrated, the mirror when if flips up provides a major obstacle to light, so light would have to bounce into the viewfinder and off the pentaprism in such a way so as it manages to bounce around the mirror and find its way to the sensor or film. The front of the mirror is sealed by the bumper completely on film cameras though there is a gap in it on DSLR's and the rear is sealed by the hinge, and if you look there is such a small gap on the sides as to almost not be worth mentioning. Especially considering the light would have to bounce off the inside of the mirror box to get back to the film or sensor even after getting around the mirror, and the whole interior of the camera is a matte black designed not to reflect light.
Basically unless you have direct bright light shining on the viewfinder at night (block it with your body or an object without touching the camera) or have extremely high ambient light such as using a super high count ND filter during the day, you don't really need it (and probably not even then from the limited testing I have done).
I would agree. The biggest impact I find is on metering when using a tripod. Once you get focused, it needs to be covered before you press the release, or the camera might just change exposure on you