Are attempts at custom mission making with the demo assets meaningless ?

[Petike]

You know, I'm not sure which retailer I should go for. I'd go for Valve, but I dislike the idea of being always connected to the internet or dependent on the site for running the game.

Do all retailers have this as their policy or is there some exception (where you pay for the game, download it and "keep it forever") ? Thanks in advance for any replies.