You don't have to have a chain keeper OR a real single-ring if your chainline is perfect. I mean *perfect*. Line it up with a straightedge and use whatever spacers or respacing you have to do to get it within .1mm.
After that, get a dedicated single-ring on the front.
The OP is talking about a 1x8, 1x9, 1x10 or whatever setup, not a single-speed setup.
But you're right, no chain keeper is needed on a single-speed/fixed gear. If it is, something is wrong.