The problem isn't knowing that you are on your last ball, it's not having enough credits to buy more of them.
WGT says that they won't let you simply use any ball in your bag because in real life play players are only allowed to finish a round with the same type of ball you start with. But if this were true, they would not simply replace your ball with the generic free ball but would forfeit your round because you didn't finish with the same ball.
Therefore, if wgt is saying it is ok to finish the round with the generic free ball, it should be equally ok to finish it with any ball in your bag. If not, they should forfeit rounds were players run out of the same ball as started with. They should also then offer the opportunity to purchase the same type of ball rather than forfeit the round AFTER you have used up your last ball.
What they currently have is a compromise which simply does not jibe with real life rules. I would prefer that if you run out of one type of ball, you are able to replace with any ball in your bag.
Or perhaps an additional compromise. For ranked or scored rounds follow the strict rules of playing the round with only one type of ball (forcing players to pony up the credits at critical moments). But allow replacement of any ball in practice mode, alt shot, or any other mode that does not figure into your average.
I also would like to be able to look at my equipment prior to starting a game and see how much life is left in any of my partially used balls. It makes no sense that I have to start a game just to see how much life is left in a ball, only to have to forfeit that same game in order to switch balls prior to teeing off.