Still till this day mates and I play Halo CE on our PCs. Microsoft just have to look at Remedy's success with their port of Alan Wake to PC to see that there is a market there ready to exploited. How can Microsoft say no to more money? If Microsoft gave 343 Industries the go ahead (or any other developer for that matter, since Gearbox ported the original Halo to PC) they could port Halo 3 and Halo Reach (and if they wanted to ODST and Wars, but personally not the biggest fan of ODST) to PC and throw it in as a bundle with Halo 4 for a little bit extra money, or with a Collector's Edition.
Another idea is putting it up on Digital Distribution sites (Steam, Origin, Games for Windows Live, Green Man Gaming. GOG.com, etc.), as previously mention Remedy made back all of their money they spent porting Alan Wake to the PC in 48 Hours and then went on to make a large profit. Then there are the prospects of future sales, you just have to look at Age of Empires III success on Steam. Age of Empires III is a fairly old game, but was constantly at the top of the Top Sellers chart on Steam during its initial release on Steam.
If the Console fans are worried about 343 Industries shifting resources from the Xbox 360 version (as many fans would be) then do the PC release three-six months after the initial release on the Xbox 360 as many other companies do (e.g. Ubisoft). This will give the developer porting Halo 4 time to do the game justice for a PC release e.g. HD Graphics, remappable keys, FOV slider, no 30 second FPS lock, etc. Or even give it to an entirely different developer.
There are many, many fans of the Halo series on PC with a market ready to be exploited by Microsoft yet I don't understand why they don't. I understand the Xbox 360 is near the end of its life and we're expecting Microsoft to announce the next Xbox soon, so Microsoft is still trying to shift as many units as possible before the release or announcement of its next console, but PC Gamers like a certain type of gaming and they're not going to shift anytime soon (this is not me personally as I have - not own - access to all three consoles in this house).
Microsoft, you have an incredible grip in the OS space with more computers running a windows based OS than any other. That is a huge demographic of gamers you can exploit for money. Then you have to also take into account the mobile space with Smartphones and Tablets. You're already doing that with the Xbox 360 and if you do the same for the PC that is more money in your back pocket as many PC Gamers like Windows Phone 7.5 and are waiting for Windows 8 Tablets and Smartphones (We'll leave the desktop alone for the moment) to use in tadem with their PCs.
I see no reason as to why you wouldn't port any of the remaining Halo games to PC other than the fact you have no interest in the gaming PC Market. However I am pretty sure it was stated back in 2010 that you wanted to lead the PC gaming revolution and you admitted that you had done very little in supporting the PC community.
http://www.mcvuk.com...d-pc-revolution