The demo shelf I drilled doesn't have anything to lock it in place, possibly one of the holes wasn't dead straight as it fits on quite snug. As eracode says, my backup plan was to drill up underneath and put a grub screw into the coach screw on the proper one.
The full metal guide would definately be more accurate than this one, it can twist/flex a bit. Once the hole has been started it has the sidewalls for support, its always been 'accurate enough' for me.
The benefit of the portable guide vs a drill press is that you can drill holes into the wall square with this, eg when you cant take the workpiece to the press.

