YSK because the first hot spell is when most people discover their AC unit is broken.
Several years ago I waited to turn on my AC. This is how I found out about one of the busiest times for repairmen and had to sweat for two weeks. Now I do a test run a couple of weeks before it heats up. Same goes for the furnace at the end of Autumn.
Edit to add: YSK line (Rule 2)
This is true of car ACs as well. Many modern cars with climate control do use the AC to some degree in defrosting, but some use periodically will keep seals from drying out and causing leaks.