as neuro and brian pointed out, the purpose of f/2.8 or wider apertures isn't necessarily to help you shoot in lower lightI don't want to get too metaphysical here, but I think this is not a case of "purpose" so much as a case of "camera manufacturers dealing with the realities of camera tech vastly different from film."
If we could get the additional sensitivity of lenses faster than f/2 (or whatever that limit is) back, you'd better bet it'd appear. It's not an intentional limitation of the lenses.
That being said it's still pretty important for people to know about this. Lens manufacturers don't have much incentive to tell people about it, which is regrettable, but apparently it's not caused any lawsuits yet, so...It's interesting that the third party lens manufacturers don't bother with f/1.2 lenses, though, isn't it?