First of all, I'd like to withdraw my remark about qualifying as a "good" reason. In the context of Brian's first response to the initial question, I'm afraid my comment might be interpreted as being sarcastic, and that was not my intent at all. It was a gratuitous statement at best, and I apologize.
About the DX mode, my technical expertise is not broad, to say the least. So perhaps you could elaborate on the reasons for your scepticism about the D800 not performing adequately as an APS-C camera? (If its an appropriate thread for this D700 forum, of course.)
As for my experiences using it in DX mode, I haven't done much shooting in it yet. Neither do I now have a dedicated DX camera to compare it with. However, I'll try to compare some identical photos taken with it in both DX and FX modes, as well as with the D700.