Ditto for me with Soldier of Fortune II. I had so much fun creating a custom mod for that game (NUTSHOT!), and just getting people playing together. I swear I played that game for 7-8 years, maybe longer.
The API I think would open up a lot to people with C/C++skills (or those willing to learn), like for example I want to be able to add additional resources into galcivII. Everything I've read says you can mod the xml, but it's hard coded somewhere that it won't allow you to add anything beyond what is existing.
What about a true total conversion mod? The "total conversions" that are out there now for GC2 are nicely done (and it shows a lot of hard work was put into them), but technically it's just a face lift on what's already in GC2, space combat in a single universe. What if someone wanted to do a mod that converts it to a combo or gc2 and alpha centuari? (not saying it's feasible or even possible, but modders have been known to pull of some crazy stuff).
If we had an API, maybe we could add alternate dimensions that you could jump in and out of or battle in. Maybe come up with a stargate tech for some civs, instant transport to/from any planet that has that tech. What about stable wormholes? That could add a new level of depth to the game. As Marvin stated, improved AI behavior, perhaps hacks to get around bugs, etc. The possibilities are only limited by the amount of exposure the API allows.
I understand that it would be a lot of work on the part of Stardock, but I personally think it would pay off in the long run when the game (in one form or another) was still being played and purchased years down the road.
And since Stardock hasn't replied to this thread, I might as well ask another crazy question, any chance we could get an API for GCII? 
P.S. Yes, I *am* certifiably insane and have all the appropriate papers to prove it. Wait, I have to go now, the men in white coats are coming...
