There is a difference between a butt pack and a lumbar pack...old school butt packs were meant to be carried over the butt and under a regular pack...they were always a little uncomfortable but if you had to ditch your main pack you had your essentials in your butt pack (almost always carried with suspenders, ala LBE/LCE). On the civi-side, a simple "waist pack" pretty much filled the same role, but are not designed to carry much weight.
Lumbar packs actually carry above the butt and into the small of your back...these are much more comfortable and usually have a more ergonomic belt system. As already noted, lumbar packs keep your back cooler and unless you have shoulder strap/strappets, your back stays much cooler with better air flow and you have a better range of motion for your arms (such as for climbing, scrambling and bouldering).
Regardless of whether you use a small pack or lumbar pack, a good, cinching waist belt is a necessity to keep either from bouncing around, stabilize your center of gravity and put the majority of weight on your hips (where it belongs) instead of your shoulders.
There is a breaking point where the lumbar pack needs to move up to a shoulder pack. Weight is probably the biggest factor in making that decision. Some just don't care for how the lumbar pack fits, but as long as you don't put too much weight in them, they are much more effective at off trail rock climbing/scrambling than a regular shoulder pack.
ROCK6