>Can anyone see an advantage the D3x has on the D800<
1. The D3x battery life slaughters the D800's 2. The D3x grip does not cramp one's fingers (for certain photographers) unlike the D800 3. The pentaprism housing on the D3x does not interfere with the Nikon tilt/shift lenses, unlike the D800 4. The D3x has two CF card slots instead of two mixed card slots
>The D3x was never up to the D3 or D3s in low light capability, and neither is the D800 it seems.<
It may seem that way, but the reality is different. Downsampled to the same resolution for a like for like comparison, the higher resolution cameras tend to win.
Comparing an older generation camera against a state of the art camera seems an odd thing to do to me. All technology is superseded in time. The D800 is no more immune to this truth than the D3x before it.
>But it was a great landscape and studio camera.<
Was? The D3x remains a truly outstanding camera. Period.