There was a recent aikiweb poll on whether aikido was violent and I believe I was in the minority "yes" side - the arguments for it not being violent are the same ones being presented here. Notably, that it is "intent" that determines whether something is violent or not. This is an attitude I cannot disagree with strongly enough. History is littered with abhorrent things done in the name of good intentions, fervently and honestly held by the monsters in question. Violence is the net result of your actions, not your intent (although you can of course intend to "do" violence).
To take the example offered by Mr Tibbets of "inadvertently" breaking an arm when defending yourself. The defence itself is violent, whether the arm is broken or not. You're referring to justified violence (also a very personal and tricky definition). Many spiritual leaders (such as Ghandi) preached non-violence, which meant allowing violence to be perpetrated on you. A nice defence performed with the best intentions, that leaves the attacker non-the-worse for wear is still you committing violence.
Are martial arts violent? Yes, but the intentions of the practitioner for pursuing the martial art aren't necessarily violent or even linked with any violent intent. I also don't think learning MA can subjugate violent tendencies, but they can give a person more discipline and options in how they cope with internal and external violence. Having said that, as there are so many different paths to spiritual growth etc. other than MA, I'd be surprised if anyone practising MA for the spiritual side didn't also have a more "violent" reason for persuing this path...