I'm in much the same position as you, D7K & D300s for primarily landscape shooting. When hiking and trying to go light, I carry the D7K. When shooting closer to my car, I more typically work the D7K with a wide-angle and D300s with a telephoto to avoid switching lenses.
I have pre-ordered a D800e and as appealing as it is to have a light body and battery compatibility with the D7K, I'll miss the speed of the D300s too much for bird shooting, so I plan to sell the D7K on arrival of the D800, which should also fetch more money used.
To your question - should you buy the D800 for primarily DX shooting? I'd say that's an easy no. It's a waste of money; $3000 for something which is not really an upgrade to your current kit.
If you mean to shoot the D800 FX when weight is not an option, and use it as DX in order to lighten your lens setup for hiking, maybe that's reasonable, but it does mean maintaining 2 different lens kits. Also, I believe you'll find the D800 viewfinder quite claustrophobic in DX mode compared to a viewfinder specifically designed for DX. Maybe a magnifying eyepiece could help that.