biggest reason is, free game engines suck.
theres a reason they are free. no one would pay for those engines, because a lot of them have pretty bad support, and are lacking a huge amount of features. they are usually the equivalent of 5-10 year old software.
if people aren't willing to pony up the cash for the great games that come out of valve... odds are they wouldn't be interested in golden eye anyway.
not to mention being able to attach ourselves to the Source brand gives the mod a bit of credibility as a project. without that, and with us just going off with a free engine, i doubt we would have had the success we have today.
Nicksters decision to chose the source engine, i think was the best decision for the projects success. yea newer engines today can beat it, but they weren't around back then. and we are still waiting on crysis, and unreal 3.0 and we are coming down the home stretch on this project so for when the mod started, source really was, and still is the best choice.