I bought the doom games again because they are now DRM Free!!! They have given me so much joy over the years that I'm happy to re-buy DRM free versions.
I'd do the same for purely idealistic reasons, but I'm not made of money - and as mentioned before, Doom has been open-source for years anyway. If I'm buying any Beth-Soft game here that I already own, it's Morrowind - because that game is still closed-source and has (to my knowledge) never been released before without any kind of DRM or copy-protection. Here's hoping CoC: Dark Corners, Dishonored, New Vegas and Wolfie TNO will someday follow.
I have played the DOOM games most months since their release 20 years ago and even though I just purchased them less that a month ago on steam as I did not have the DOOM master levels, I have repurchased them as to me having them DRM free and complete with the setup.exe and pre fixed missisng key for final DOOM was worth it.
The fact that I did not own the Quake expansions and the purchase price on gog for the DOOM and Quake bundle is the same as just purchasing quake1 and the addon on steam also helped you also get the CD version of Elder Scrolls: Arena
for free (only the floppy disk version is available for free else were), as I have not tried it I can't say if its any better than the floppy disk version. You can find out about the differences here [url=http://elderscrolls.wikia.com/wiki/The_Elder_Scrolls:_Arena]http://elderscrolls.wikia.com/wiki/The_Elder_Scrolls:_Arena[/url]
But you are correct you don't get anything for the DOOM games that you don't already have from the stream versions and free 3rd party downloads to get the final doom missing key patch and missing setup.exe file that is available with the shareware version of doom anyway.