I'd always heard the same about clinchers... I think a lot of it is just repeated "knowledge" from a bygone era.
In my experience I've never had or seen a clincher "blow off the rim" like many of the arguments on the net against them seem to think is guaranteed to happen, but I could see a scenario where you flatted on the track towards the top, and as you cut back down it rolled it off and possibly toppled you and/or ruined your rim... in which case if you had a seriously glued on tubie (I know some folks love that carpet glue still) it *might* stay on and not roll off. That's pretty corner case.
*Of course for racing on a smooth track you can't get the same pressure in a clincher, and currently no tyre manufacturer makes a track specific super low crr (on par with vitt pista evo's/olympics/dugast silks/etc) so if you want the fastest out there, you do still have to go with a tubular.