An yet, all polls on this forum about whether people would actually want/use/pay for a MP GC turn out "NO!" by a factor of 9 to 1.
Also note that Civ4 changes its format so you can play an entire game in less then 4 hours. This was "required" to make the game suitable for MP play, according to the Civ Developers. 2 hours or less is the preferred time frame for an truly strategic MP game.
MP has a lot of different design concerns then SP play. StarDock has already stated that to make a good MP game, the SP game suffers. And that's true in Civ4. The AI is very under-developed for the game. But that's alright, according to the Civ Developers, because the Civ community will develop a good AI, in about 2 years. So that means that the Civ developers made the conscious design decision to short-changing their SP player base (you know, those customers that have been paying Sid's bills since he formed MicroProse), by at least 2 years, in favor of making the game a decent MP TBS game. Of course, the AI in Civ has always sucked, so it isn't really noticable. But Star Dock's AI is known to be of the highest quality in TBS games. Cutting it down would be noticed by the reviewers and customer base. That would generate some badwill with their customer base, so it might not be as attractive to Star Dock as it was to Take Two and Firaxis.