stfas: While I agree in principle, I agree with Ira in this situation. Petitioning for moddability (which will benefit the company in the long run, and which is something Relic is know for in DoW, Impossible Creatures and Homeworld) is different from demanding that race X or unit Y is included. One is saying "give me X or I throw my toys out of the pram and go and find another game" and the other is saying "give us X and we'll build a community to add Y and Z to the game and extend its life"
Also, just look at
Counter-Strike. That started as a mod and became a whole game with expansions. Where would Valve be without it? No-one has quite gone that far with Dawn of War, but depending on the flexibility of DoW2 then they might be able to (and they have done with Homeworld, which is still going).
Ira: I guess most people probably will still buy it, just because they like the game. As you said, though, without the mods then the game life can be shorter and less enjoyable. I'm still borderline on the whole "buy it or not" issue.