I use the centre point because you know where you are with it and in marginal situations supposed to be better. For motorsport, it's not a problem most of the time as the focusing distance is usually fine in the middle anyway for me.
Depending on the subject, I sometimes change that, and photographing my daughter's dog the other day I let the camera choose with multi-point in servo AF. It worked fine, but I think centre point would have been equally good looking at the final images but I wasn't sure if he would run out of middle frame at the time.
Bear in mind how AF works. The sensitive area is quite a lot bigger than the spot in the viewfinder. You can see what it actually is by lining up a small near target with a clear edge, against a background also with good detail for the AF to lock on to. As you move the focus point closer to the near object, it will suddenly latch on to it and focus, and it will hold on to it until it falls outside the sensitive area again and revert to the background.
When you have subjects at different distances within the sensitive AF area, the camera will tend to favour the bit with most contrast that is easiest to lock on to. But if contrast is similar, it will go to the closest part. In practise, I find it tends to favour the closest object within its sensitive area, but sometimes it gets drawn to something else and goes AWOL.
Basically, I think centre point is more reliable if it doesn't compromise your framing. If it does, then use multi point which should be equally good for most motorsport. But if you were shooting football from the side line and wanted to concenrtate on a particular player, multi point would be hopeless if other players kept moving in front of the target and triggering one of the outer AF points.
