100% coverage is a big deal for people who don't like to crop and like to know exactly what they're getting before they shoot. With digital it's easy to check the LCD and reshoot, but back in film days it was always surprising what snuck into the corners of an image. Those of us (myself included) who printed "full frame" (which back then meant a ground out negative carrier ) always had a few frames ruined by something that JUST got in the edge where we couldn't see it.
As for the actual SIZE of the finder, not sure if it's age because I'm "only" 31 and have (corrected) good vision, but I really prefer larger finders (like in the 1 series, 5 series and D700). I will say for ACTION shooting in bright light, the smaller crop finders can be nice because it's easier to check the 4 corners when they're closer together, so for very fast framing, small can be nice, but for what I typically shoot (candids and landscapes/travel) the larger finder makes all the details easier to see and get right.
If you've ever used a non 35mm system, part of the joy in MF and LF shooting is just how big the finder or groundglass is.
honestly though, this is similar to screen/tv/monitor sizes. A few people will tell you smaller is better for whatever reasons (cost, space), but MOST people will tell you bigger is better.