I've always found that the experience of some degree of dissatisfaction with the technical results of certain photos I've taken under certain conditions, usually less than ideal conditions, has been a driving force and a motivation for me to upgrade my equipment, both camera body and sometimes lenses.
Such dissatisfaction usually relates to unwanted noise and a lack of sharpness. An increase in pixel quality results in reduced noise. An increase in pixel quantity results in increased resolution, all else being equal. However, both are inter-related in the sense that a noise-free image can be sharpened more in post processing, and an image which is already acceptably sharp, but rather noisy, may not require further sharpening which usually has the effect of increasing noise.
I can understand that someone who is very pleased with his D700 or even D7000, and also pleased with the results on a 17" print, might see no reason to upgrade to a 36mp camera which he thinks is more appropriate for 24" x 36" prints or larger, which he has no intention of printing.
However, there is another more practical aspect of upgrading to a higher-megapixel camera, which is often overlooked. If the increase in pixel count is very significant, as it is when comparing the D700 (or D7000) with the D810, then there are amazing flow-on benefits relating to the enhanced quality and increased, effective range of focal lengths of one's existing lenses, in relation to that personal standard of acceptable quality on a 17" print.
All of one's prime lenses effectively become short-range zoom lenses with a fixed maximum aperture. In relation to the 12mp standard of the D700, the D810 with 50/F1.4 lens attached will effectively become a 50-86.5/F1.4 zoom. In other words, the crop factor of 1.73x, for a 12mp image, is greater than the crop factor of the DX format, which 1.5x. The Nikkor 80-400 zoom on the D810 (in relation to that 12mp standard) effectively becomes an 80-692mm zoom.
Of course, one might raise the legitimate point that a 12mp crop from the centre of a 36mp full-frame sensor will not be a sharp as the uncropped image from a full-frame 12mp camera with use of appropriate lens. That's true, just as it's true that a 36mp image down-sampled to 12mp will be sharper than the same image taken with the same lens on a 12mp camera.
However, if such increases in sharpness are considered to be of little consequence on a 17" print, because they would require inspecting the print from a very close distance, then such reductions in sharpness should also be of little consequence.
