I agree as well, but I believe that the game should have one 'canon' ending that you get the first time through, but when you choose to replay the campaign, you have the option to enable alternate endings, in which case, you are given a non-canon ending depending on your actions in the campaign. By having the canon ending the only one available on your first time through, players who rush through just to get the story don't get an alternate ending and end up confused over what happened.

If this is implemented it would increase the replay value of the campaign, but also leave room for a continuation using the official canon ending. However, all of this assumes that the campaign is very similar to Starcraft's, which doesn't seem to be the case from what I've seen of the Terran campaign; it seems very nonlinear, but I would assume it ends almost the exact same no matter how it's played.