The sheer amount of different pre-order DLC for Dragon Age Origins made it impossible to play with them all without pirating it. Of course, as it turns out, they're all junk that you wouldn't want to play with anyways. (Aside from that one with the golem character. That was a pre-order thing, right? I forget.)
If a pre-order gives me an unfair advantage over what the single-player game was intended to be, why would I want that? TF2 items don't do that, of course, but I don't play TF2 anymore anyways, so that's no incentive.
... I could probably sell TF2 items or something, but that's hard to arrange AFAIK and Valve probably doesn't approve.
Also, games that say this also just piss me off (Looking at you Max Payne 3): "WE INCLUDED A DLC WITH MAPS AND/OR MULTIPLAYER STUFF THAT PEOPLE WHO DON'T PRE-ORDER DON'T GET AND HAVE TO BUY SEPARATELY!" (I still haven't bought that. I know they're going to release more DLC, and then there's going to be a new version of it released that includes it all, for the same or lower price than the game originally released for. Now Borderlands 2... Pre-ordered that... Not sure I'll like it, but hey!)
Not sure how much of this they're doing for Firaxis' XCom game, since I haven't looked at the pre-order page at all, though. This conditional pre-order bonuses thing is kind of ridiculous though. It's like they went "Hey that kickstarter stretch goals thing is popular, let's do that but instead of enhancing the game further if there are a lot of pre-orders, let's just add more pre-order bonuses because that will be really easy and won't delay our release date at all!"