Hey, this may be the first valid reason to prefer a double that I've heard.

Sheldon's BB Sizes page lists only one comparable instance of double vs. triple Q for Shimano: 9-speed Ultegra. Double is 147, triple is 156. That's a shade over 1/4", and about a 6% increase. That doesn't sound to me like much of a difference - or at least not enough to outweigh the advantages of a triple - but then I don't have bad knees.