The variability of the cap value and the IC trigger point makes the spread too wide to simply pick a set of components. We had to calibrate them with a resistor selection, the pot just eliminates the need to test resistors before soldering them in.
Obviously, it could have been done with a 555, but as Stan pointed out, that would have run the current consumption up quite a bit. Although there are doubtless tons of other ways to do this, this is the way we have. 
