Quite frankly, strategy guides became useless after I found out about Gamefaqs.com so many years ago.

There's little chance that COD4 players would pay for a map pack, because if you get bored there are more than 500 custom maps available for download, and some of them are of excellent quality.
With MW2, however, there's no choice. If you're sick of the original maps... you have to pay.

Most popular games get a GameSpot/IGN visual guide in addition to text-based ones on GameFAQs and the like, only games that are very new or obscure lack a satisfactory guide in those places.
If the Steam overlay didn't have a web browser the Prima guide integration would certainly be an appealing option, but as it is you can almost always find some suitable information (even if it involves going to the forums to see if someone else has asked about the same thing--it can still all be done in-game thanks to the new overlay).
If the guides were $5 I could see this being viable (especially if the "Digital Deluxe" versions included them for free), but $10 is well within the price of a full game on Steam, especially with "midweek madness" deals and whatnot.

Well I think nobody bought it on PC. At least nobody on my Steam friends list did.

The MW2 map pack has been in the Steam top sellers since it released.

I don't remember intelligence being a factor, I just noticed you say "nobody bought it on PC" and yet tons of people did.
Same thing happened with MW2, they said no dedicated servers and the clan crowd said the game would bomb... it went on to sell a confirmed "millions" on the PC and was in the Steam top sellers for months, still is a lot of the time, and is a verified success. The reaction from the clans? "Well no one smart bought it."
That doesn't matter. We ALL, unfortunately, have to face the fact that the majority of gamers are not super hardcore forum posters who care about these things. They want quick gameplay experiences with an easy to understand goal and not too much challenge. They want Oblivion, not Morrowind. They want Modern Warfare, not Battlefield. They want Halo, not STALKER.
They want something that tells them what to do as soon as they don't know.
Way of the world. Sucks.

So if you only play mainstream games (like TES3/4 and F3 :p) you are set. But what about the less popular ones?
I am about to reinstall Silent Storm. Every time I plan to play this, I get annoyed at how hard it is to find images of the skill trees (admittedly, I think those might be on the disc :p). Not the best example, but worth considering.
Like I said, this isn't something I would go out of my way to grab. But if it was a pre-order incentive, I very well might. It isn't a must-have, but it also isn't the evil and worthless thing people want to say it is.