What Chris said. In the event of a rollover stock belts allow your upper body (and head) to move. A proper harness locks you in place. If the roof collapses, well...let's just say it wouldn't be pretty.
Maybe Auto-X speeds are low enough that there is zero chance of a rollover. But if that's the case, I probably wouldn't be interested in doing one
Actually I'm not an auto-x fan because of the lack of actual time on course. I'd rather pay the extra money, drive to the track, and get multiple sessions under my belt. But ymmv...