In reply to Danny Brown:
At my local walls they use Gri-gris in pairs (one pulling in, one holding the brake rope) and only do the taking in, not the lowering down. This is just for larger groups of non-climbing novices e.g. school clubs, charity climbs etc. Only the instructors operate the cam handle since this is the only bit where correct use is absolutely required (whereas correct use is required for all actions with a plate-type device).
There is a girl guides-type group who turn up occasionally. Every time I watch them, I am bl**dy glad they are on gri-gris... human error is far more common than equipment failure after all, even with experienced people!
AFAIK neither wall I visit hires out or teaches the use of their grigris (unless you ask I guess); they are only for groups who are not being taught to belay. Are you referring to the situation above (non-climbing novices using grigris) or people being taught to belay only with a grigri?
You do seem to have a bias against gri-gris, which are after all the standard gear for sport climbing. Used correctly, they are safer, as is any other assisted braking device. Used incorrectly, any device is not safe. Used badly (e.g. hand off dead rope, letting go completely) gri-gris will generally still function. Yes, you can drop people lowering them if you are careless, but this is easily avoidable.
PS this is two walls in Devon :P
Post edited at 22:29