I agree about the audience being less energetic during matinees. I went to the Avenue Q matinee back in March I and thought the cast gave 110%, but the audience really wasn't that into it. During curtain calls I was all ready to do the standing ovation for the final bow, but no one else in the theater stood up. I was in the front row (yay for lotteries :) ) and would have loved to stand and applaud the cast were it not for the fact that I would be face to face w/ someone's crotch, which is just a little too awkward for me, especially when I would be the only one standing.
Anyways...
Matinees always seem to be a little bit more casual than evening performances for audiences IMO. I think there's something about dressing up and going to Broadway for a "night out" that helps put people in the mood to see a show. Going at night and seeing the marquis and Times Square lit up, and eating at a fancy restaurant is all part of the experience, well, from a tourists' perspective anyway.
Does the sun really rise in the east?
Does the earth really spin around the sun?
What's it matter in the least?
What's real to me ain't real to everyone.