I would much rather people take out their violent urrges by say playing a first person shooter rather than going out and shooting people. Many people, who are not completely insane, can relieve their violent urges through means other than physical violence. The same goes for sexual urges too. This is my primary reason to allow the viewing of any sort of pornography. The makers of violent pornography are a completely different story (a point on which I agree with you whole heartedly), but we're talking about viewers.

The long and short of it is, if violent crimes can be prevented by allowing potential offenders to satisfy themselves in a virtual manner, that is a very good thing.