The major selling point, to me, is the USB dock:

  • Being able to change AF parameters on the lens is great, especially if used with bodies without AFMA. This seems to ideally suit the target market of the 17-70. For the pricier lenses or better bodies, heck, depending how they've done it, it might even be better than the in-body AFMA: e.g. 1Dx style rather than 7D.
  • Self-upgradable lens firmware. Anyone investing $$$$ in a 120-300 f/2.8 can have some confidence that if there is an issue with future bodies, the lens can be upgraded to suit.

Of course, it helps that they look the part, too. If they do an update of the 70-200 f/2.8 in the new style, with USB, I'd be sorely tempted.