MIBCTGOLF- I agree with you only to the point that it should be allowed in practice games only. There are far too many sandbaggers and multi-accounters here who would love the opportunity to fleece you of credits by showing they're playing weak equipment at the start only to switch to the better stuff after the game starts. In games that actually count for average or are for credits there are very skilled players who would tailor the ball used for each hole. Added distance for longer holes, more spin for stopping power on short-sided pin placements, less attributes for less wind effect on the ball.
To only allow it when a ball runs out still lets the first group set you up and suck you in. Buy the cheapest balls, count the hits on the last ball and save it with only a few hits left. Use what's left to set up an unsuspecting player in a credit game.
Practice games are something different. With nothing on the line it could be combined with the mulligan feature to be a learning tool. See how different balls react while hitting certain shots. Learn which balls work the best with certain clubs.
Knowing that WGT imposes the "one-ball rule", it's very simple to make sure your bag is properly stocked before you tee off in the first place.
The rule, by the way, is not limited to the PGA. Virtually every regional tournament the SCGA hosts here in southern California for members representing different clubs imposes the same rule and most of us are often just recreational players.