I find the top screen useful for things like timelpase or long exposure, so I can monitor what the camera is doing but without using the rear LCD.
A trivial point but I prefer the blue light of my EOS 3 than the orange or green light of DSLRs.
I think they are in some ways redundant, the always on rear LCD, as is the way with live view cameras (I know you can disable it) means there is far more info there than on the top LCD, and as both require you remove your eye from the viewfinder I have reached another conclusion...
EVFs can be as good for general use. The resolution is there, things like focus peaking are better than trying to focus manually using plain matte focus screens as come as standard, low light, perhaps not as convincing as signal noise on the preview image can be distracting, as can the multitude of overlays etc..
The idea of having no mirror blackout is potentially great for ai servo tracking etc... maybe a pellicle with switchable EVF overlay? Or something like the design used in the old panasonic L1 / Olympus E330.. similtaneous optical and electronic paths...
Or just ditch the mirror altogether?....
Different debate sorry.
Top LCDs were far more useful to me in the film days. The only time I use them now is when I'm helping my wife learn her camera.