> Given that the batteries weigh a lot more than the motors, I would have thought that motor efficiency (which scales battery size) was much more important than motor weight.

I guess that depends on what kind of airplane you are making. If you're just making the same kind of airplanes we've been making with ICE, but with electric motors and batteries instead, you're probably right.

But if you're making an electric airplane from scratch, there's a lot you can do if you have a really light motor, which can drastically reduce drag.

Look at Maxwell X-57 for instance: https://www.youtube.com/watch?v=-HvZ7c0F9ik

If you're going to have lots of motors on the wings, they better be as light as possible.

I'm guessing the increased efficiency from a design like that can easily be as important as the efficiency of the motor itself.