Some of Canon's (or Nikon's, Sigma's. etc) best lenses are barely able to keep up with some of their better cameras in resolving power. It would be no surprise to find many P&S lenses are not up to the level required for the 12 MP many P&S cameras now have.
In other words, it is just marketing but it is a single number of "goodness" that the consumer can use for comparison. If I bought a 6 MP a few years ago and now they have a 12 MP it *must be* twice as good so I will buy it right away and get twice as good images.
It is not all myth though. I have a 3 MP Canon D30 and an 8 MP Canon 30D, using the same lens there is a difference in image quality. No, it's not as much as expected from almost 3x the pixels but it is there.