Am I the only RTS player on Earth that regards linear, "story based" lead-you-by-the-nose campaigns in RTS games as boring, pointless, and unplayable? (1)
Strategy games simply aren't good story telling devices IMO. (2)
(1) I would have disagreed with you 100 times over had I not played through the Battle for Middle Earth II campaign. Just thinking about it makes me want to hurl. Playing the 'sandbox' mode is tons of fun usually, but the single player campaign is just... it cripples the game as a whole, in my opinion. It goes so far bad that it actually un-teaches you to play the game, all the while waxing 'neo-dramatic'.
(2) I think they CAN be. For example, Starcraft and Warcraft (III) have deep and complex story lines with real characters and (mostly) interesting conflicts. These two also use story elements and mission goals to 'teach' you certain aspects of the game, like microing a specific unit or something. So they're a double-good thing. Games like Age of Empires have their own sort of appeal in that you can make 'historical' campaigns with them.
Command and Conquer has always been very weak in story line, though I definitely liked Red Alert II. However, they have a lot of flair in their single player game so it keeps it at least interesting.
If you're resorting to chug up a plot from the black depths, then you're better off just making a really kickass sandbox. I thought the Galciv II campaign was mediocre at BEST, for example. However, the sandbox game makes it basically negligible because it is enormous in its depth, basically accounting for the whole game.
As to whether or not Sins should have a story-line 'campaign' - well I think if it's gonna be something GOOD then go for it. You really have to have a Blizzard mentality with those kinds of things - if it rubs you the wrong way, trash it. Whether or not people admit it, I think they'd rather have no fleshed out story line than an afterthought or a poorly-developed one.