I never liked the AF-on button. It is poorly located and just does not feel comfortable. However in some situations I find it indispensable and I have it programmed as shooting menu B in my D300. Obviously it is not advantageous to engage the autofocus every time you push the shutter button. Try getting a duck in the reeds in focus and then push the shutter button only to have the autofocus go into a search. So what I did was set up a front button as a focus lock. Once I achieve focus using the shutter release, I lock it by jamming on the front button. This works great where you have to recompose after doing a precise autofocus and on subjects that are moving but no out of the focal plane.
It fails when I am using a remote shutter release. In using a remote release I would force a refocus on a subject already in focus. I have the back button focus set up as menu B along with my HDR bracketing settings.
The only disadvantage of programming a front button as a lock is when I am shooting with flash I have other uses for that front button. That is to stop the camera from performing a preflash once it has already done one. So when shooting indoor events with flash and I want to catch fast action I set up the front button differently. If all of this sounds complex try reading the 1,000 page manual and remembering all these things while under pressure. But there is a huge payoff in getting the difficult shots.
Edit to add. I would much rather have my thumb solidly holding the camera on the rubber grip than pushing a button. Especially as i often shooting over water. Plus it is not a stable grip.
Edit 2. I never shot a photo with autofocus for my first 12 years of doing photography. I have yet to find a camera that can read my mind and put the focus where I want it by pushing a button. Alas my eyesight is not as good. Autofocus slows me down and I find it more difficult to keep the focus exactly on the eye of a flying bird rather then the bill or near wing. F
