The thing about all the powder being consumed within 18" of barrel may be true with standard type target/game loads. I think the short barrel trend was mostly a matter of handling ability for cops in tight places than for hunting. Shotgun makers picked up on the short barrel handling abilities in tight places and translated that to thick cover in hunting situations and offered that as an attractive option with the thought in mind that nearly all the ballistic properties were kept intact. The slight velocity loss wasn't much with the loads of the day and the ranges most loads/chokes were capable of putting together a complete pattern.
The newer high velocity hunting loads that have been developed in the last few years that most turkey and predator hunters are interested in seem to need a bit more tube. I've noticed a trend away from the shorter barrel lengths on turkey guns and more emphasis on mid-length barrels. Seems there are fewer 20" - 21" turkey hunting barrels and more 22" - 24" options than in years past. The interest in shooting at longer ranges for waterfowlers, turkeys, and now us predator hunters have spurred the direction of the newest high velocity shotgun loads with the large shot sizes. There may not be any difference in patterning ability between a 18" barrel and a 24" inch barrel, all things equal. However, there will be a difference in individual pellet energy. The ability for enough pellets to reach the target with enough energy to penetrate deeply for sure kills is what the "long range" shotgunner of large critters strives for. Thus the success of high velocity loads, plated lead shot, and new shot material that is denser than lead such as Hevi-Shot.