Well I think it's a great idea, imagine being able to cut a straight line without a straight edge. Or, to be able to cut materials that won't take a mark or would be spoiled by marking. Or, stuff like velvet that is difficult to cut with a knife.
It all depends on the implementation. If the laser is out of alignment it's game over. If the laser line is so bright or fat that you can't tell where the center of the line is it won't be much help. But if done right I'd buy it.
