Another suggestion to come out of the \quit_if discussion was that someone might want to send a return code other than 0 back. Something like this:
\quit 4
or even:
\set exit_code 127 \quit :exit_code
This isn't a personal need of mine, but I figured it was an idea worth discussing on its own.
\quit exit_code is better - if we define some special variable, then we have to specify when it should be used and when not. Taking value from command is clean without any another questions.