Ape's execlp() tries first the program name passed to it (be it relative or absolute), and if that fails it prepends "/bin/" and tries again. It ignores environment variables entirely.†

Rc's path variable allows you to easily tell rc to check the current directory when looking for programs to exec. Doing that with bind after each cd would be clumsy. And if your working directory is a remote server, you can set path to just /bin so that you aren't statting the remote directory before each exec. Inferno's sh does use a path variable, but it is typically left unset and the default (/dis .) is used.††

† See http://plan9.bell-labs.com/sources/plan9/sys/src/ape/lib/ap/...

†† See /usr/inferno/appl/cmd/sh/sh.b:/^runexternal

rc has $path because rc predates Plan9, it first appeared in tenth edition Unix.

Inferno's shell doesn't use a $path variable.