I'm a little torn about this. I personally enjoy 3 set matches more than 5 set matches on average, simply because some "easy" matches get dragged out too much in the BO5 format, and sometimes the losing player can sneak in a set somewhere along the way which makes the whole ordeal even more extended. Also, I dislike the idea of tennis becoming a sport about endurance more than skill/ability. I feel like the whole sport would be more "to the point" and pragmatic if we just shortened it a bit. 3-4 hours for an average GS match is just way too long for my taste. Makes it hard to sit through the whole thing (people have got other things to do).
On the other hand, the most awesome, memorable matches in history are BO5.. would be sad to miss those completely. In the end, it should be decided by the male players and the fans/broadcasting companies. Pretty much everyone except for BJK.