During the exposure, the reflex mirror (the 'R' in dSLR) is flipped up and completely blocks light entering the viewfinder from reaching the sensor. If your camera is leaking light during long exposures, you should send it to Canon Service, because light leakage sufficient to affect an image directly means a defective camera.
No, not quite. Normally the mirror blocks enough light from the viewfinder, so that it is not seen in the picture. But if you use a filter like ND 3.0 or IR 720, which lets only 1/1000 of the light pass, the light from behind can be clearly seen in the pic, even with a non-defect-cam (on an IR-image it looks blue).
I've not done IR photography with digital, but I have done a fair bit of shooting with ND 3.0 filters (I have them in 77mm and 82mm sizes for various lenses), and I've certainly never 'clearly seen' any effect of light through the VF (I don't use a cover, because I'm shooting manually with the 10-stop ND). When I shot IR-sensitive film back in the day, there was some light leakage - but the build quality of the cameras I could afford then was nowhere near today's standards for prosumer bodies.
Perhaps you could post an example of a shot taken with and without the eyepiece covered, during a long exposure with a 10-stop ND, to illustrate the point? I'll try that, myself, too.
The main reason for the cropped camera is the extra reach for most people and a DX mode would solve this problem.While I agree the reach is an important aspect and a crop format would be very helpful, I don't think it's the main reason. I think most people buy a crop sensor camera because it's cheaper. If you can afford both and then you choose the crop camera, sure, but most people would buy FF if they could, IMO.
I do agree that the main reason most
people buy APS-C is becuase the cameras are cheaper. But the question was for people who have a 7D and buy a 5DIII, would you sell the 7D - in that case, assuming the 7D isn't being sold only to cover the cost of the new body, the answer might be different. I have both a 5DII and a 7D, and I'll replace the 5DII with a 1D X or maybe a 5DIII (depending on the real AF spec, mostly), but either way, I'll be keeping the 7D - in situations where you're focal length limited, a crop sensor is the second best solution (the best being buy longer lenses, but over 400mm gets $$$$, and the crop sensor is better than cropping a FF image down). It all depends on the use case for the images, of course. I've done detailed comparisons of the 5DII cropped image vs. the 7D in focal length limited situations, and the IQ is basically a wash. But the 7D image is still 18 MP and can even be cropped further if necessary (which it often is), whereas the 5DII image cropped to the same AoV is only 8 MP, and while that might be sufficient, cropping further isn't such a good idea.
A 'DX mode' is only a solution if the FF sensor has sufficient resolution to support it - the 36 MP D800 yields a 15 MP DX image, whereas a 22 MP 5DIII would only yield an 8.5 MP image. Also, a crop more presents problems for framing the image - with the masking, the VF image becomes very small, since there's no way to magnify the cropped portion to fill the VF.