Never in my wildest imaginings did I think that the modification would be anything other than a 'proof of concept' for tactical simulation. We had talked quite a lot about the features of Doom that made it a tactical urban trainer. The opportunity to modify the elements of the game gave us the ability to simulate live fire situations. Once we put an actual fireteam together and ran through the exercise, we were pretty happy with the result. People were utilizing the knowledge they had been trained up with and observing the results. Soon, after demonstrating the mod in many places, we posted it to the web to facilitate distribution. Because of the publicity it generated, people in the outside world began to download it. Simple answer, no, I had no idea that this would catch on. I just thought it was a good way to explore some tactical questions at the fire team level.
If you like tactics and strategies in your Doom game, another good bet would be Twilight Warrior, which blows Marine Doom out of the water in my book.