GOG's game files are well set up, in my experience, and GOG is by far my favourite shop for PC games, since they are DRM free and work no problem (so far!).But admittedly I just run the games the way GOG intend, I've not tried to recreate the original disc files or anything. Still, given that GOG do so much for the gamers' good, such as setup the games to run well on modern systems, and include so many extras where possible (i.e. art work, and official soundtracks), then it does seem a little strange that they don't offer the original, untouched installation files for anyone who also wants them.
Is there any legal reason why this should be the case (I can't see what, but then I'm no lawyer), as even if the demand is small, it would be a nice incentive to buy the GOG version of a game rather than from another company.