when they started classifying oils, the Pour Point was considered representative of engine operation.
W ratings were once based on pour point (and viscosity in Cp), then evolved to "borderline pumping"...these were found to be again, as per kschachn's post not to be representative of how oils behaved in engines.
So relatively recently, MRV (Mini Rotating Viscometer) replaced these measures, as basically how easily the oil "falls" into the oil pump pick-up tube (Dynamic viscosity maximum, and absence of yield stress)...pour point is viscosity independent, it can still be pouring, but not able to be sucked up (think straw and thickshake).