I know I'll regret sticking my nose in the discussion between kittykat46 and JackBak.
I think you are both right but using a different "stack." If you average, which is what most astrophotographers do to reduce noise, the brightness does not change but the signal to noise gets better. If you add (which I never do) then the brightness does increase. Normally lit objects get brighter. Dim objects which can't be seen in the short exposure will not magically appear when added as kittykat stated.
Note: Adding in camera requires a dim subject so the sum does not exceed the maximum pixel value. Adding in software just requires the program be able to scale large sums properly. Usually that means 32 BIT math routines.
There is also BIT resolution to consider. Let's say the short exposure only has 2 bits between the brightest and dimmest parts of the nebula. It will look pretty awful with only 4 brightness levels. No amount of adding multiple frames will fix that. GIGO!
Using an exposure 16 times longer gives the nebula 6 bits between the brightest and dimmest parts of the nebula. This allows smooth transitions between the wispy parts of the nebula.
My conclusion? Use the longest exposure you can reasonably take and take lots of them. The better amateur astrophotographers take 10-20 minute frames for many hours.
And I just have to add one of my images of Nebula M42. K100D through FL 2000mm telescope 2 minutes * 42 frames.