But my theory is that if you are prepared for something bad, then it doesn't tend to happen.

I have used this concept throughout my adult life. It's good to be prepared, but it can also be used to control what kinds of things you have to deal with in life if you strategically prepare for things you wan to avoid happening. It has served me well over the years and it does have a pretty good track record of working out.

This is how I think of it too. Its certainly helpful to be able to do things effectively with both hands, and doing so increases the thickness of the myelin layer sheathing the nerve wiring for that portion of the system. Imagine having two, equally good routes to get somewhere. Having that doesn't get you there twice as fast, but it means problems like losing a limb, or a stroke won't be as catastrophic.

I guess I kind of seeing it as you've been curling all your life with a single bar, but then you switch in some isolated curling. You find that one was depending on the other slightly without knowing. Now you've got both trained and up to the same strength. So in a way, you actually have become slightly stronger over all.

Also they say left and right sides of the brains tend to do certain things. Like the 'left' side being the artistic side. I was wondering if the left and the right hands were predominantly handled by different sides of the brain. If so you're actively more of areas in different sides of the brain that weren't before by isolated training / experience gaining. Therefore maybe, by doing more activity on that side, it's expanding it's overall abilities.