martial arts may not have been invented solely for defending or offending. There is a school of thought that says that the Bodhiharma "invented" a set of exercises resembling yoga to improve the conditioning of the monks at the shaolin temple so they could medidate longer. (over simplification, but the basic idea). according to this "theory" MA developed as an offshoot.
It has been well documented as to the benefits mentally and spiritually of MA training. The Marine Corps and the Army have recently adopted/revamped/placed new emphasis on MA training, not so much to train people to be effective at hand-to-hand, but to improve their warrior spirit.
I think self defense is more of a byproduct. Frankly if this is what you are concerned with, I can think of many more effective ways to defend yourself from knives, guns, to pepper spray. Much easier to learn and much easier to be effective.
Aikido is named aikido for a reason. It is a "DO" or way art. Focus is on developing and refining you as a holistic being. Some might enjoy it for the physical aspects, some for the mental, others for the spiritual. The fact is, that they are all there regardless of what you personally get out of the art.
Frankly I think it is more of a rip off if all you get out of it is some rudimentary fighting skills.