Let me clarify: Its use as a slicing blade isn't what's limited. It's that it's pretty much what the katana's utility is limited TO.
You have a single edge which limits the sort of cuts you can perform. Thrusting is awkard because of the curve of the blade, and is also further limited by blade geometry. A sword WILL NOT cut through plate armor. I don't care what magical properties you think Japanese swords had. Steel does NOT cut steel (I think the Mythbusters proved this pretty conclusively) in this manner. The katana is a very specialized sword. At best you could cause blunt-force trauma, but against high-quality plate (which used ridges and roping to further stiffen it) you're going to ruin your edge before doing significant damage to an opponent.
For sheer flexibility and versatility the katana is hopelessly outdone by the longsword, and its pure cutting ability isn't so far behind the katana as you might think. The two edges of the longsword provides more versatility in the cut, and the guard is as much an offensive tool as it is to protect the hand (hooking, tripping, striking, binding the blade, etc). The pommel is also just as often used offensively as it is to counterweight the blade (where do you think the verb "pummel" comes from?) The taper of the blade makes it ideally suited for the thrust which, combined with half-sword techniques, makes the longsword idea for close-quarters fighting against armored opponents where precision thrusts can defeat the best suit of armor.
