Dull blades will shorten the grass, but it also damages the tips of the grass and splits the ends of the grass, which in turn makes the grass tips brown.
We put on sharp blades every day and our lawns always look greener than our competitions lawns. You can see an exact lot line when one is cut with sharp blades and the other is cut with dull blades. Dull blades don't cut the grass, they just beat it to death. Try shaving with a dull razer blade and see how you like it.
Maybe this is one reason some of you say you can't cut a lawn while mowing fast. If I used dull blades, I couldn't cut fast either. I carry extra blades just in case they happen to dull up faster one day than they normally do. I feel the blade, if I don't feel it could cut my finger, it is time to change them.