I'm just glad to know I'm not breaking the rules when I'm actually playing golf when I grab a crappy ball from my bag when I'm teeing off on a water hole.
As for the whole "one ball" non-sense: The term as it applies to WGT is just that. If the game used that rule, players would get DQ'd if they run out of purchased balls, whether it's through wear or a lost ball. As someone said, WGT is being "kind" by allowing players to continue with the starter ball.
Not that I'm about to run out and test this theory, but - Could a player close the window for the round after they're last ball is gone, then buy a sleeve in the proshop and return to the game and be able to continue with the same type of balls instead of reverting to the starter balls? If that works, that is SO not PGA rules or real life for that matter :)
For what it's worth, I would agree that if you should have the choice to use a different ball from your inventory if your current "sleeve" is emptied. It seems to make sense and most closely mirrors the "written" rules of golf.