It sets expectations. If you get a BG, you should expect to be able to ride…and if you don’t, because you were kind of “promised” you would, they compensate you. But the BBG is there to let you know as soon as you get it that there is a decent change you won’t be able to ride…and if they never get to you, you shouldn’t expect anything.
If they made them all BGs and not BBGs, then they would still make them small to reduce risk to themselves…but if they then get through everything without a hitch, how do they handle the rest of the day? Just do SB? Is it not more fair to give priority to those who were there early to get a BBG?
I think, considering, Disney is going things fairly well this way. It may not be perfect, but I think alternatives have worse flaws.