>What is the purpose of this 18-35mm? .... > I don't understand what gap it is >filling.
Here's how I see it..... In the past few years Nikon has methodically updated all their best selling AF-D FX lenses, both zooms and primes, to the G type (with AF-S).
With this latest addition there are now three complete ranges of good quality FX zooms bridging the most popular 18mm to 200mm range : - entry level set = 18-35, 24-85VR, 70-300VR - midrange f4 set = 16-35VR, 24-120VR, 70-200VR - the f2.8 trinity = 14-24, 24-70, 70-200 VRII There are also f1.8 and f1.4 G-type FX primes in the 35mm, 50mm and 85mm focal lengths.
Note there is no such cohesiveness and order in the limited range of DX offerings.
All DX and FX G lenses are compatible with all DX and FX camera bodies, unlike main competitor Canon which has the EF/EF-S lens incompatibility problem between the APS-C and full frame bodies. .
Finally, because they all have AF-S, Nikon now has the opportunity to introduce a low priced entry level FX body ala D3200/D5200. By removing the in-body AF drive, replacing VF prism with mirrors, removing the top LCD, substituting lower cost AF & shutter modules and going all plastic for the body, you could probably sell an entry level FX camera for less than D7000 prices.
As for the future of DX...Nikon has shown commitment to DX by continuing to develop DX lenses but note these tend to be lower cost models of late. The DX market share is being nibbled at both ends by new innovations - Mirrorless interchangeable lens (MIL), Electronic viewfinder interchangeable lens (EVIL) cameras and bridge cameras are making inroads at the entry level end while lower priced full frame cameras like D600 and Canon's 6D are competing with top end Dx/APS-C models. In both Canon and Nikon cases, the top end DX/APS-C cameras (D300s and 7D) are getting long in the tooth with no real clarity on a successor. the D7000 is also getting long in the tooth. When there is more profit to be made elsewhere, would you prioritise your R&D efforts in the DX direction?
I suspect in future that the DX range will have only 3 model lines; existing D3x00 and D5x00 with the mid-range D7000 and top end D300s being consolidated into a single enthusiast/prosumer line...or maybe even dropped altogether.