The camera should really give one all four options:
AF-S with shutter priority or focus priority, and
AF-C with shutter priority or focus priority.
Most often I'd like to have AF-C with focus priority but that option isn't available.
I've got AF assigned to the OK button (on a K100D) in order to decouple it from the shutter button. I understand that the AF button on a K20D/K-7 can be used in the same way.
By using separate buttons for focus and shutter release one then only needs AF-C, as one just keeps the OK/AF button pressed as long as the camera should keep tracking (short press = AF-S). No need for AF-S as such anymore, but sometimes I'd like to use focus priority while tracking with AF-C, i.e., let the camera prevent me from taking a photo before it obtained focus.