I guess thats my point though, now DX sensors are up in the 20MP+ region crop-ability is less of an issue AND ISO performance is getting more comparable so the benefit cases are getting more extreme - as you say, pushing into darker scenes. I love FX though - it provides a set of great lenses of which I can use the centre and get great performance ;-) I only have 2 DX lenses, Sigma 10-20 for WA and the Nikon 17-55/2.8 - all my others are FX which suits as I mainly do sports, aviation and wildlife.
Very true. My D90 was only 12MP so it makes a huge difference (the cropping) with 24MP on FX. My D90 still takes great pictures in good light. 12MP is more than enough with that camera. As you say, the benefit case seems to be wanting to get better images in more difficult situations. At least based on the specs we see, the D500 should be the best Nikon DX made yet in that regard. I'm guessing results in most situation will certainly rival some FX cameras. You also illustrate another use case in that you focus on sports, aviation, and wildlife where the reach and FPS (and better ISO) are all plusses. I gravitate towards landscapes and adventure where there is not as strong an attraction to the DX plusses for me.
I'm certainly not a more MP is better guy but I am a bigger sensor is better guy. All things being equal, bigger sensors (or bigger film negatives) will collect more information. FF sensors have made their way into DX sized and Mirrorless sized bodies. I hope one day soon we see an MF sensor making its way into a D750 size body. The Pentax 645Z is not to terribly far off (but maybe still along way to go).
BTW, the D5 (of course) looks like it will be an ISO monster but a good bit out of my budget. A little bit heavy for me and what I like to do as well.