Your post is sensible IMO T-Man, I also think unreasonable to ask the BP team to even consider the option of substituting tracks for two main reasons, one that which Battuta alluded to, it will ease some fans, it will irate others (so why bother?), two it's just too much work and in some cases almost impossible (the Icarus scene is synchronized with its soundtrack for example, and so is Freedom Fighters).
I think this discussion is nevertheless very useful for it brings to the table a question that we as members of a wider society should think about, namely what constitutes copyright, intelectual property, fair use, their limits, common sense, etc., and as members of a modding forum, what kind of standards should we uphold, what is reasonable or not, etc.
In the case of BP well, it's hard. The ambitions of the bp crew were immense, and it's hard to emotionally convey all the mind states without borrowing the very best that is out there. I have a curious fact to share. You do know that in Tenebra we get in Universal Truth the 2001 soundtrack
Requiem (youtube link here:
http://www.youtube.com/watch?v=GgqI32JX_jY), and it is the perfect track at the perfect spot. What you might not know is that Kubrick himself used this music without permission from its author, Ly Ligeti (who then went on to successfully sue him and eventually settled).
So this is a very old conversation indeed.