When Lennon became the guardian of the "essence" of rock and roll, McCartney was only implicated in doing music, which was always censured by the puristic critics of rock world. Plus, McCartney got an enormous succes in the 70's; so many of critics only could make their critics more intense than before. So McCartney was a insubstancial balladmaker and Lennon was a real rocker. This is completely nonsense, the man who made Yesterday and Band on the Run, Helter Skelter and Soily, Eleanor Rigby and Live and Let Die gave us some of the finnest pieces of popular music (Lennon as well) and is absolutely underrated.
PD: Have you heard his discography with Wings and after? There are good songs, poor songs, bad songs, grat songs and master pieces; just try it.
It is true that Paul McCartney has written some pretty sappy songs he has also written some amazing ones. Also, his musical abilities span far beyond rock and roll. During the Beatles he wrote some very cool songs in the style of the 20's and has composed 4 albums of classical work, no small feet im sure.