<p>With practice, it becomes second nature to use the thumb to control focus with AF-ON (at least on my D300), and the index finger on the shutter release, it is not really much different from doing the half-press to focus, and then the full-press to trigger the shutter. Using the AF-ON button and setting the AF mode to AF-C, gives you instantaneous control over AF-C, AF-S or AF-M without using any other button except the AF-ON. If you don't press the AF-ON, no change in focus, or you can use manual focus on the lens. If you press AF-ON and release, you get AF-S. If you keep AF-ON pressed, you get AF-C. You don't have to worry whether you accidentally let the half-press go, particularly when you want to reselect what your exposure was also based on. It is great to have exposure and focus decoupled. <br>
<br />I too am from the old MF days, and if there is sufficient DoF, I too fallback to focus and recompose as my preferred method, since it saves having to twiddle with moving the focus point around. It works just fine with the AF-ON button.</p>
<p>As others have said, AF-C and subject tracking is exactly the same whether you use the half-shutter press or the AF-ON button to engage the AF system. The difference is that the AF-ON button puts you in control of when the AF system is engaged, whereas with the half-press you have no choice but to engage the AF system before the shutter activates unless ou have set a second button to choose manual focus.</p>