What do you mean by "rotated correctly in the mouth?" I saw in the opening post that you mentioned the correct orientation of the mouthpiece, but it seems to me that tilting it back with a keeper would actually negatively affect the position of the joint. Of the full cheek snaffles I've seen (which, admittedly, is only a handful), the mouthpiece is always perpendicular to the cheeks, just like in an eggbutt or D-ring. You don't rotate those for everyday use....and if you rotate the full cheek using keepers, aren't you going to bring the joint down so that it breaks over the tongue rather than straight back? It seems that this would also influence the ability to do any sort of lateral reining, as, unless your horse is carrying a perfectly vertical headset, you'll be pulling across the joint rather than breaking it.... (this all makes sense in the picture in my head, but I'm afraid it's not coming out in words very clearly)
The safety thing I understand, so I always try to be hyper-aware when using my keeper-less full cheek.