Of course it depends on the camera model. The exposure time on a camera like OM-4 would not be affected by light coming through the viewfinder because the sensor is inside the mirror chamber and once the mirror is up, the cell is looking at the film and updating shutter depending on light off the film. But the ESII meter cells look at ground glass from up in the prism area... These cells would see light through the viewfinder when the mirror is up - so that camera needs the viewfinder blind feature to keep viewfinder light from altering exposure.
Originally Posted by E. von Hoegh