What do *you* need though? Are you expecting a lot of low light? Using it with a crop sensor? Don't need to travel light? F/2.8 anyday, it provides more flexibility. I have been using the f/2.8 with my d7100 for the last 6 months and am absolutely loving the images, but unless I *need* the IQ or DOF, in good light I don't notice any appreciable difference vs my 70-300vr at comparable zoom lengths.
I think for a lot of people the question is not only why the f/4, but whether their needs justify the fixed f/4 apeture and build quality vs the 70-300 vr. If you're careful and keep it at or below 200mm, it is also fantastically sharp, but you have a less sealed and less well built lens.