Apple laptop lineup is getting as crowded as Dell and Lenovo these days. I'm surprised they are building such a large MBA, it might be curtains for the 16-inch MBP in that case.
It's about as crowded as it was in the aluminum PowerBook G4 days when you had the 12" iBook, 14" iBook, 12" PB G4, 15" PB G4 and 17" PB G4. Now they have the 13" MBA, the 13" MBP, 14" MBP, and 16" MBP. The only model that really needs to go is the 13" MBP as it brings very little to the table over the MBA. A 15" MBA would make a lot more sense than the 13" MBP IMO.
Many people are speculating that the 15” MacBook would replace the 13” MBP in the product line.