It would *GREATLY* add to the immersive experience and get a lot more sales if there was a campaign
A campaign with a good story is one that always has me liking a game. I actually have the opposite opinion of Helioforge - but then, everyone is perfectly capable of thinking for themselves.
1. I never actually owned the original C&C on PC (I got the PS1 version of it, and the awkwardness of the controls meant I could never really play the game and like it at the same time), but my first experience with RTS was indeed with Red Alert. Though I was suggestible at that age, I remember wondering how they managed to get a camera into Stalin's office and seem to record every major briefing of a war that I had not yet realised never happened. I still play Red Alert - and only for its campaign. The skirmishes aren't bad, it's just that the campaign is just
so much better.
2. C&C Generals ZH is pretty much the opposite. I thought the campaigns were pretty rubbish, but the skirmishes are nothing short of awesome, in my opinion. There's actually a motivation for me to go out there and become an expert at each and every one of the generals, just because I think the game is so good; I know some hated it, though

3. Nexus TJI I think is very good, but I rarely play it because of its awkward control interface (IMO). I think the reason I like it so much is because of its tremendous campaign. This is one of the few games I have where I think the game is excellent solely because of its campaign (of course, graphics, sound, and the Master & Commander feel in the game help, but would be pretty pathetic if one could only use a skirmish function).
4. Homeworld! Nuff said. The reason I think HW1 is a better game than HW2 is because it has a better campaign, though I think the gameplay in HW2 is more conducive to skirmish/online play. The same goes for the Ground Control series.
5. I haven't played Halo 3, and I thought the second one fell into the same sort of trap that Matrix 2 fell into. I thought the first one functioned very well as a standalone game, but I agree with Helioforge that there's just no real reason to play it. I don't understand why some game developers don't include AI function in skirmish (non-existent in Halo), LAN, and internet games, though there's probably a good reason. The reason I think it's such a good game is just because of its story - I think it's pretty pathetic as an FPS on its own. Still, I don't regret having bought Halo (I shouldn't actually be comparing it with the others, given that they're RTS's and RTT's).
6. Emperor: Battle for Dune was one of the last Westwood games, and it was good enough for me to play through both the campaign and the skirmish multiple times. It combined, IMO, the sheer awesomeness that Generals has and the compelling story that RA has.
I think the point I'm trying to make in all the above points (especially contrasting 1, 2, and 5) is that a game without a campaign can be very good, and a campaigned game without a skirmish function can be very good, but if a game can do both well, then it reaches into the space of the Homeworld series. Sins has made a fantastic game (and on my "awesome scale" it measures just about better than Generals and WiC). But the expansion with a good campaign could make it so much better! The immersive experience in a game gives it so much respectability. This game is so good already - why not improve on it further with a campaign? Of course, it has to be a good one (observe Generals with its weak campaign), but good campaigns aren't defined by how repetitive they are.
IMO, one plays a campaign for a story, for an atmosphere, for immersion. One could also play it as an extended tutorial (considering how in most cases the difficulty moves from low to high), but it's the story!
Oops. It's a diatribe! My bad!