First, a no-brainer: having your feet attached to the pedals, whether via clipless pedals or clips and straps, means you don't have to worry about your foot coming off the pedal during a high-effort spin.
Second, the talk of "extra effort pulling up" is bogus. A kid learning to ride a bike with pedals quickly (or not so quickly...) figures out you can't push with both feet and expect to get anywhere. You need to lift the back foot to make the pedals go around, and that drives the bike forward. Don't lift the back foot too far, to fast, as then your foot comes off the pedal and you crash (unless the kid's learning with clips and straps, yikes!), but even a child can learn to un-weight the back pedal a bit. Another term for "un-weight" is "lift."