I think that you're confusing things if you think that internal strength is all about fighting or self-defense. Of course, the applications are obvious, but what we're talking about is really a superior way of moving and using your body.
For my money, it's not "outside" of Aikido at all - unless what Morihei Ueshiba was doing was "outside" of Aikido.
I also think that Mark's distinction between "inner strength" and "internal strength" is very important. On the other hand, I think that the two were intimately connected in Morihei Ueshiba's training method - so much so that you need one to understand the other.
Here I am writing this long post then Chris posts. Okay, fine...
Delete it all.
What Chris said.
Then add "because I want to be able to do what I sometimes feel from some of my sensei without waiting another 20 years to get there.". Impatient? Maybe. Or could we consider the possibility that there may be better models available now to understand what was being done hence a maybe more efficient means of transmission of this one aspect of a larger art? I don't think the art *itself* is missing anything. However, people do differ considerably on what the art *is* and whether it has been faithfully transmitted.
If the previous is correct, well, then maybe I can in turn become a better teacher for the few students I teach. I do feel a responsibility to my art. To transmit it the best I can. And if I find another way to convey something to a student that is of value to them, well, I'm a happy guy.
But in the larger picture... If you feel things are just ducky the way they are, more power to ya. I shrug a lot about this stuff. Whatever floats your boat as my dad used to say. Or, another way of putting this point is to point out that nobody ever asks why I like playing tennis. Or why I try different ways to fix my swing. I know someone who does Aikido as a form of dance and movement. He is about as martially effective as a dead bird. But he's okay with that, he's getting what he wants from what he does. Great for him. Just not for me, however. We can, however, coexist without the universe imploding on itself.
The answer to me is quite mundane. Because I'm interested. Because I like it. Because it's fun. Because it is stimulating. Because maybe it will give me a way to be a better student of something I love. Or maybe make me a better teacher in turn.
Aikido as an art for me isn't a question of absolute slavish devotion to some idealized icon. To me it's about simply going where the path leads me. And doing so sincerely.