I can't understand how his (McCartney) new material  
is so pitifully weak and boring. McCartney needs someone he respects to  
tell him to cut out the shite (Lennon seemed to be the only one who  
pulled this off)....

I think I shared this here once, but George Martin also had that authority with McCartney... During the "Tug of War" sessions Martin actually stood up to the Macca ego and after auditioning the songs he had slated for the album indicated which ones were good and which ones weren't... Also, during the recording process he made Macca perform many track takes and is quoted to have said (paraphrasing) "We have to do this right so that the final product makes you look good"... So yes, besides Lennon, George Martin is one of the counted few who could/can afford not to be one of his "yes" men...