Fenixp: , it's the fact that videogaming history remains obscured in a labyrynth of copyright issues, and some might never actually resurface.
Crosmando: What do you mean "resurface", all these games are freely available on abandonware sites. Having games in the public domain is not a guarantee that they won't fall into obscurity, either way sites will host them for download.
In fact, there's an argument that having them officially licensed and re-released on GOG does more for old games being lifted out of obscurity, because GOG is able to package them with DOSbox and get them running, also GOG comes with us, the community. Having a game in the public domain means private individuals/websites will be hosting the old game files, and they aren't likely to set it up in DOSbox or whatever like GOG does.
There's no evidence that having games in the public domain would preserve them any more or any different than they currently are by dedicated abandonware sites, public domain guarantees nothing - only dedicated fans who want to preserve old games will do it - GOG included.
PD binaries and art gain you quite a bit but not necessarily as much as you want.
PD source, on the other hand, gains you everything.
There is a parallel effort to the development of better and better emulation/etc. like dosbox that could be happening if people were able to work on updating the game itself to run in modern environments (see Ur-Quan Masters for example). That would be a good thing.
Not to mention that learning from history is always good. The computing world is very bad at this sort of historical sharing and learning (not just in gaming).
Have you seen the people who "fixed" the Atari ET game? They have an incredible write-up of what the bugs were, how they went about fixing them, and what trade-offs they had to make in the process. The fact that people with that skill, ability, dedication and time exist in a marvel and they should be praised but it shouldn't require people and effort of that caliber to resurrect and fix and old broken game. I don't know what the source code to the ET game looked like but I have to imagine that fixing the bugs in the source would have been much easier and faster and would have required fewer trade-offs.