You have to admit the rivalry does..
add to the fun of it. Every team has it's rival and we "the Yankee fans" are lucky enough to have the Red Sox as they are a strong team with very devoted fans, as we are. My boss is a Red Sox fan, we have fun with it. Yes, most Yankee fans are hoping that the Red Sox lose. And believe me, this year we have had to hear it from the Red Sox fans, right from the start of the season when we struggled the most.
Maybe I fall into the 10% you discussed. To tell you the truth if you live in NY you will find that most of the Mets fans hate the Yankees as much as the Red Sox fans do. My family is split, half Yankee fans and half Mets, the difference is that the Yankee fans will route for the "NY" Mets as long as they are not playing the Yankees. The Mets fans route for the Mets and who ever else is playing the Yankees including the Red Sox.