First of all, I have to admit, if you take all your photos in RAW, then the tone curves don't really do anything since you'll be able to alter the photo in Photoshop with the exact same effect.
However, here's the kicker: If you take JPG's, the custom tone curve applies the curve when the photo is in 16 bit mode before it is converted to JPG 8 bits. Altering a jpg afterwards will not give you the same results.
One person in particular in the DPReview forum tried to apply the exact points in Photoshop to a JPG image in normal and compared it to a photo taken with the custom tone setting, but the results were not the same (I believe this has to do with the bit depth or perhaps Nikon's histogram is mapped differently than Photoshop's).
The only thing this tone curve was meant to do is add the equivalent of +0.5 EV without having to set the EV correction in the camera, which many people already do. The reason why I chose +0.5 EV is because I found that actually applying +0.5 EV created gray card photos with histograms centered at the center of the histogram, where they should be in the first place.
I am not claiming this curve does anything else than set the brightness level to what you would expect out of any other camera, without blowing out highlights any more than if you simply used the normal tone setting. That's all. It's not magic.