Where it would be helpful is in swapping out reservations. What are my chances of dropping a reservation I know will be gone in a minute for this other reservation?
If there is only one room left, that's a different decision and different "odds" than if there are several. At that point, I just want to know what the odds are of making a successful transition. Knowing how many rooms are available would be extremely useful in that regard.
Example. Feb 25th open Poly booking. How many people held their breaths that the move would be successful and they wouldn't end up both without a new reservation and lost the old reservation? Apparently, this shouldn't have been a great concern, and an inventory count would have shown this.
Honestly, I don't see a downside to this for Disney? I do, however, see an upside. It's a simple software change and the result would be happier customers.