Any kind of carry, concealed or otherwise, will be a compromise between security (as in preventing loss of a handgun) and speed (as in a fast draw). In the case of inside the waistband carry, it sometimes (but not always) seems more secure even without the absence of any kind of thumbsnap or strap. Part of that is because when the handgun is in position, it presses against the rib cage and that helps to keep it in place under normal circumstances. It also depends a little on how deep a IWB holster holds the gun. For a revolver, the belt, if one is worn, definitely keeps it in place.
I recall an article in a gun magazine years ago about someone who lost their stainless steel Colt Officer's ACP out in the woods during the winter, with snow on the ground. They did find it in the spring, still in excellent condition, save for the sights. Personally I would have been too embarrassed to write such an article myself, about losing a pistol in the woods.
So, in addition to not shooting yourself, rule number two is to not lose your gun.
