In the summer it's tougher for airlines to predict the number of seats they can sell to avoid oversell situations because a significant number of seats may have to fly empty due to heat restrictions on the day of the flight.

If you're flying short trips on RJs with less than 60 seats, and have no status, I'd consider forking over the $12 to get a seat. At the very least, check in as early as possible to reduce the chance of IDB, as you'd be entitled to no compensation in this case.