IMHO the real 'killer' app for a standardised API would be to allow the various products you are using to interact in real time. So in Roll20 you choose to attack with your two handed weapon and it requests from HeroLab what the current dice roll should be. Or an area effect spell such as bless can be broadcast to all players.
The individual stat block is one aspect, but you also want to transmit state transitions.
The point also being that you want the manufacturers to concentrate on their specialities.
However such a standards implementation is not an easy task. It requires a certain amount of generalisation whilst at the same time keeping focus on the key requirements to make for cool gaming. This is a balancing act which most standards hardly ever get right. As lonewolf-rob says it would place a big drain on most games companies with limited resources.
So ultimately this would have to be a labour of open-source love, which just might gain some momentum and then some sponsoring.
But I would be happy to take part if there was some kind of working group.