To me, it's a question of what I would want to see if I was at the track.
If I'm at home, I don't care if the race is run on schedule or the next day; that's what DVRs are for.
If I'm there, it's a question of whether I want to see a race run in the rain on the scheduled day, knowing it's likely to be less than competitive and chock full of cautions; or whether I want to pay for an extra night in the hotel in order to get a better race. Me, I'm opting for the hotel stay and a likely more competitive event.
Besides, I'm a sucker for going to races on 'make up' days. If race days are a different atmosphere from your daily life or other sports events, postpone days are an almost surreal experience. The crowds are usually smaller, the parking lots less crowded. The fans share a hardcore race fan, 'I see you made it back, too' attitude. Most of the vendors are gone, but so are the ushers and such; sit where you want, nobody cares. I love a 11:00 or 11:30 start time, leaving plenty of time to drive home after the race ends.