It is also important to note that Frenzy raises the inclination for the target to attack anything, not specifically you, which can cause funny effects with the Radiant AI (NPCs killing each other, etc.)
This spell is useful if you need to kill someone, for a quest or otherwise, but don't wish to be prosecuted for assault or murder, as killing someone in self-defence is not a crime.
A good frenzy spell can also be a good way to train blade, blunt or hand-to-hand. It can be used to anger an NPC that can't die (a character needed to complete a quest), and once they begin to attack, you can fight back without being arrested. About 20 seconds later, the NPC should get back up and you can repeat.