I can kind of see the logic in a 15" macbook air. If you take away the optical drive and have a sealed battery you could easily make it so the 15" could hit 8hours like the 17" and also as probably reduce the overall size a bit as well. I personally see the 15" screen as the ideal size for a presentation laptop when theres no projector etc.
They could probably keep all the ports (split onto both sides now though) and give it the macbook airs slanted shape or make it overall thinner (just

)
For me personally I hardly use the optical drive on my computers, most of my backups are on a server and I wouldn't mind the lack of it in a laptop if it gets me more battery life. I'm more than happy to carry a portable drive with me for the rare occasions its needed - I generally know when this is likely to happen anyways.