Someone thinks one particular thing is super important who, I guess, may not have actually done that much photography. In the photos I take I basically never have time to look right to the edges of the frame, so 100% coverage is pointless. Having a viewfinder that facilitates quick, definitive focusing, however, is something else altogether and is very important. The internet gives everyone a mouthpiece, and advice is mostly given in definitive terms, whereas the answer to most things is variable. In photography, some craft things, like archival fixing, can be done ‘right’ (your negatives will last) or ‘wrong’ (they will not last). Almost everything else comes down to preference, or, as some call it, ‘art’.
A 100% viewfinder is nice for me on the rare occasions that I have a camera on a tripod or I am photographing something that doesn’t move, or where I have a composition whose main elements are right at the edge of the frame. None of those things happen to me very often.
