Some of my thoughts:
1. Sometimes mappers can lose map sources, and this can be a problem for mapper himself. I know real examples when losing sources demotivates mod authors from continuing their work at all.
2. Sometimes author is not against of using of his work, but he doesn't have map sources already, so he can not share them with anyone.
3. Sometimes people want to change a couple of things on a map just for their own needs, not because they want to steal someone's work.
4. A lot of things on any map still can be changed via entpatch editing, decompilation just allow to work in common map editor instead of using less handy utilities.
5. A lot of map sources are released already and mostly this didn't cause using them for creating of "blatant copies", so just relax, OK? From the other side, the absence of map sources doesn't prevent people from creating different remakes and such, if they really interested.
What about other questions.
1. I didn't say that decompiler can give you 1:1 copy of map sources, but very close results of general map layout are possible, with proper geometry and texturing. Map lighting is one of those things which is nearly impossible to recreate 1:1 with decompiling, so this part will require some manual work with map anyways.
2. It will not be very easy to do, and of course it will require a lot of testing and bug fixing, so there is a question: is someone really interested in this, or it's better do not try at all.
3. The project will not be started if Unkle Mike decides that there are not enough reasons for this. So currently I just asking about your opinions, nothing more.