Why 18 35???

Price: Already been covered above.

Size: When I had a D700 I carried it in a very small bag with the older 18-35 and the 28-105. That same bag now holds a D800 and the 16-35. There is a little bit of leftover space that is too small for another lens.

The same people who appreciate the size, weight, and price of the D600 vs the D800 may very well appreciate the new 18-35 over the 16-35 for the same reasons. It probably makes perfect sense for many people.