of course I would love it. BUT Activision/Treyarch would never do this. Why? They're number 1, they are going to break sales records...again. Playing it early won't get them extra sales, in fact it could lose them sales. Part of COD releases is the hype, I gotta play it day 1, gotta get to that midnight release, etc. They build it up for months, and the fact you can't play it helps that.
Marketing at it's best. Some of the same stuff Apple does (Apple does it better, but same concepts). Super secret about new product, rumors come out but unconfirmed, big unveiling, then still have to wait before release, people line up...that could describe either Iphone/Ipad or COD.
A Demo could lose them sales, as people who wouldn't like it, might not buy it. Just like there are TONS of people every year who buy it, don't like it, and can't return it so they keep it or trade it in (which loses Activision no to little money).
To me Halo 4 has no bearing on my decision to buy COD, I have both pre-ordered but either way they're very different settings/concepts, etc. Halo 4 has more to prove, as Halo Reach was underwhelming to a lot of people. If anyone should have a demo, it should be them.
Reason Battlefield had a demo was they were the underdog trying to prove something....bring COD players to BF, a demo helps that. Try to impress CODers without them having to invest any money.
Add: Also, why would it be a Gamestop bonus....it should just be a pre-order bonus since Gamestop would have nothing to do with a demo. Should apply to any retailer that did pre-orders...Gamestop wouldn't have any power to do this, it would be Treyarch/Activision.
Any if anyone is thinking, well they let people play it at Gamescon so that contradicts your above points...well yes, but that's a tiny tiny drop in the bucket compared to the millions of customers/potential customers....they wouldn't let the masses play alpha/beta versions.