If you think about what you wrote, you'd find that it's incorrect. The key thing is that your eye is looking at an image that is projected onto the focusing screen. What you are essentially saying above is that the image projected onto the focusing screen will appear to have more or less DOF as you change the aperture of the lens (aperture of your eye) you are using to view the focusing screen with - which is incorrect (physically impossible) - you are viewing a projected image, the DOF of the projected image won't change by changing the aperture of the viewing lens.
What LittleWolf said is correct. The modern day SLR/DSLR have "bright" screens - these screens are "bright" because they scatter less light - so that your viewfinder is not dark when you have those "slow" modern kit zoom lenses. The problem with a small scattering angle is then when you mount a fast lens, the DOF from eg f/1.4 to f/4 looks virtually the same.
I personally find the DOF preview button useless on modern day SLR/DSLRs with "bright" screens, as it gets too dim when you go passed f/5.6, so you're basically left with viewing at f/4 or f/5.6. Much better off using liveview to get a real DOF view (even wide open at f/1.4) and you can see and compose something at f/45 on a bellows and not viewing some black screen.
DOF buttons cannot be used EVERYWHERE. I never had a problem using it in the day, outdoors. Never done TFCD indoors, or even TFCD at all, so i cant comment. DOF is a good to have, IMO, for those without liveview.