I believe that the "dark frame" is mainly to remove stuck pixels, which are most often red, green or blue but sometimes show as white. From my usage, the stuck pixels seem to get worse as the camera is used and heat is generated, especially with longer exposures. Humidity will also make the effect worse. In the older days before cameras had "long exposure noise reduction" as an option, the technique involved taking a shot of with the lens cap on after you were done with the shot. You then could use several software programs that are designed to blend the dark frame and the actual frame, and blending the stuck pixels out. You can also do this in photoshop with a layering technique but I never found it to work as well as the software programs. To me the name "long noise or long exposure noise reduction" is a bit misleading as the true digital noise is not really worked on that much, only the stuck pixels.
The P45+ from my usage did a very good job here, but as Brian pointed out, it has to take a 2nd exposure as long as the first up to 60 minutes. And getting to 60 minutes really required ideal conditions, outside temp not higher than 69 degrees F and very low humidity. It also pretty much took the life of one battery, i.e. two 1 hour exposures. And if you calculated wrong, and the battery ran out on the dark frame, then the entire sequence was no good. Still an amazing solution and truly a one of kind for MF long exposures.
The main reason I switched to 35mm for night work, is that you can most times turn off long noise reduction, and stack exposures, which gives you a lot more detail in the night sky. It also gives a lot more to work with in the landscape part of your shot as over an hour conditions will most times vary. Wind and clouds, wind more than anything can ruin the shot. You will pick up some stuck pixels, but if you use LR and shoot raw, most times the stuck pixels will be mapped out by LR. If you shoot as jpg, then you have to revert to the manual method to remove the stuck pixels. The other thing to remember is that most times on each shoot, you will get a different set of stuck pixels so if you take the "shoot a manual dark frame" you need to remember to do it each time. Enabling "long noise reduction" will create gaps due to the necessary dark frame (when stacking) and also the camera will eventually buffer out (Canon) Nikon has no buffer and works just like Phase One i.e. shoot one, wait one for the same time.
Again, this is only in regards to night shooting mainly to work with stars and sky. I realize that there are many other applications, studio or daytime long exposures were the P45+ is a great solution.