When Tim was designing our new kitchen table and we knew we'd need stools I said that we'd only need to get three as there was no way James was going to be using a stool without a back -'he'd fall off!' But of course James only used his high chair at the table perhaps half a dozen times before one meal he just climbed up onto the stool all by himself and proved me wrong.
It IS high up and if he was to fall off it backward it certainly could cause him harm, but as he doesn't sit up there without supervision (so far) I think the reminders to sit on his bottom and to climb down carefully will insure his safety to some degree.
You can just tell that he loves sitting up there- feeling so capable and old. He especially loves having family dinner time around that table, getting his own placemat and napkin.
And his other new favorite meal time accomplishment is drinking out of cup. I know most children learn this at a much earlier age, but I never really even offered the opportunity because our house was on the market and I didn't want to have to deal with cleaning up large sticky messes.
Despite the motherly neglect he somehow figured out how to do it himself (I think in the bath and the shower) and now gleefully drinks cups and cups of milk, orange juice and water without any real spilling.