Here's a good one According to this, interleague play nets each team about 100 tickets sold One hundred
http://blogs.wsj.com/dailyfix/2009/07/08/the-count-does-interleague-play-really-boost-attendance/
My suggestion/compromise. Have one rivalry weekend a year and that's it. If you don't have a rival then you just play whoever (too bad). My friends and I did this in fantasy football for years. Fair? No. Fun? Yes.
So the Mets play the Yankees and Seattle plays the Rockies (too bad.)