You need some space on each end of the bench (the plane has to start and finish somewhere) but the issue I have had with a 6' bench is trying to prepare longer stock. The stock really needs to be supported in its entirety if you are trying flatten and square it. If part of it is hanging off the end, it will want to flex, you will have trouble figuring out a way to hold it properly, etc. If someone is sure they are never going to try to build a dining room table, a long sitting bench, a bed, a tall shelf, etc., then by all means they do not need a longer bench. Likewise if you are doing most of the preparation with power tools. But trying to do any significant preparation of longer stock with hand tools it will be difficult if you have two or three feet hanging off the bench.