as discussed and explained elsewhere, every potential change to the GOG builds needs to be submitted for approval to the entity that has allowed GOG to sell the game in question. this process can be quite straightforward - for example, Night Dive is quite cooperative so getting System Shock2 patched up is pretty easy, but I can imagine things are much worse with a mega-company that does not really give a damn like Square.
also do note that fixing up Thief is not just about using the latest executable - there is a lot of other things that need a fix, so even if GOG would indeed incorporate NewDark into their Thief games, you would still need the patchers to get rid of gameplay problems.
Post edited March 15, 2013 by voodoo47