Can't bleed air out of rear brakes

You really don't want any of the brakes to lock up. Locked wheel= no control.
(Let's not get into the whole dry pavement skidding/ stopping distance thing. Most of the time you'll never lock up the brakes on bone dry level ground).
Gravity bleeding isn't foolproof. Believe it or not, air can still be trapped in the lines. It's a good way to start/ prime a brake line, but you can still have trapped air.