I sometimes pull up on the flats when I think of it, but it takes a lot of concentration. I often find myself naturally pulling up on steep climbs or when starting off in a high gear, basically any high-torque situations.

The main benefit of clipping in is that your foot is never off the pedal, so when you're spinning at 90+rpm your foot doesn't come down beside the pedal resulting in your smacking your shin onto it instead.