Ron Natalie writes:

There's no requirement that you can't put them in the left seat. It's
commonly done when the right seat pilot is also a flight instructor,
but it could be any legal pilot.

Yes, but I presume that outside of flight instruction, it's unusual to
put someone else in the left seat (especially if that someone is not a
pilot).

It looks like most aircraft have some things that are only accessible
from the left side, although I guess all the important stuff is
duplicated (?).

Definitely. The forces of the plane work around the center of
gravity which is usually some where around the back of the front
seat. The rear seat of some aircraft will drift around a lot
more than the front. The closest I ever came to being sick in
a small plane was riding in the back of a 182 on a gusty day,
lots of swinging back and forth.

I'll remember to avoid back seats. I can get motion sickness in a
vehicle if there's a lot of movement and I can't see outside.

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