To me, it seems highly dubious whether the sort of cover under discussion here would provide much protection at all; and I think the point is valid about sand and other foreign material getting between the cover and the camera body.
I can certainly wish for a case for my D3200 that would offer decent protection, without interfering with its use.
This is my old F2 in its case…
The F2 is a pretty rugged camera to begin with, and this case does an excellent job of protecting it, especially when it is closed. When opened, it still offers significant protection, while allowing access to all the controls that I need in order to use the camera. I only ever need to take the camera out of its case to change the film.
I long for a case for my D3200 that is this good, that would protect it as well as this case protects my F2, while still offering easy access to all the controls, connectors, and compartments that I need. Sadly, I doubt if this is possible. ON the F2, everything I need to access is either on the front or near the top. The D3200 has the screen and controls on the back as well, along with the battery and SD card compartments on the right and some connectors on the left. And I don't get the sense that my D3200 is nearly as sturdy as my F2, and that it could benefit much more from the kind of protection that a case like this would offer.