1.
I see that PrtScrn actually works when using SFML (with SDL it would not copy a screencapture to the clipboard) which is nice.
My program however has a function to save a .bmp in the application directory when 'PrtScrn' is pressed. So I don't want to use it for something other than taking a screenshot, but I wanted to have this functionality associated with it. However, since the OS PrtScrn functionality seems to still work, this is acceptable.
2.
No, but it makes for smaller filesize by a factor of 4. I suppose if it was necessary I would write my own saving and loading scheme for 1-bit per pixel which is the format for glDrawBitmap();
3.
Less convenient to deal with than with integers. Growing inaccuracy over time.