L4D has/will get DLC? :o
If that's the case, then i just might get it for the PC
Valve patches up it's games on a regular basis (not just L4D). Some correct minor technical issues, but many more fix balance issues (really important with multiplayer games like L4D and TF2). The consoles almost never see these updates.
In addition to the patches, you get free access to the DLC. Sometimes you have to pay for that extra content on consoles (or you just never get the option).
Regardless of whether you can use the SDK or not, it means you get to use the mods other people, extending the life of the game.