That just doesn't make sense. It cutting through everything makes it a dangerous, difficult weapon to wield, but in no way makes it useless. In fact, it makes the opponent's defences (and weapons) irrelevant: you just cut through it. It would be the epitome of "one strike, one kill".
But the fact that it cuts through all defenses work against you as well. Normal swords are double edged, a rapier can only pierce, and a katana is only single edged. With those weapon, it is typical to rest the dull part of the sword very close to your body.
There are two problems with a light saber.
1. Every part of the blade is an edge so you would never dare bring that blade any where close to your body. You will always have to use it with one arm stretched out way in front of you.
2. When you cut through an enemy, you might as well be cutting through air. As a result, if and when you swing fast, there is a good chance that the saber will come back around and wound yourself. Pick up a sword and try a fast slashing motion and you'll know what I mean.