Tossing aside strategy games that are strictly single-player (GalCivII, MOO, etc.), what strategy game's single player campaign ISN'T a training ground for multiplayer. As far as gameplay goes, a single-player campaign eases a player into the rules of the game, progressively teaching him or her more about the game and slowly peeling back the layers of the game during the entire process.
Remember Starcraft? You had three types of units during the first mission: marines, SCVs, and vultures. You had one resource: minerals, and you couldn't do much with the units you had. The second mission ramped this up by adding gas and firebats, and soon after you had goliaths and various defensive structures. At the end of the Terran campaign you had the entire tech-tree under your disposal, but they certainly didn't drop that all on a new player right from the beginning.
IMO a single-player campaign in a multi-player game fails if it doesn't teach the player how to play the game. It definitely also exists as a device to tell a story, but whether a developer takes advantage of this is up to them.
I don't really care who Chris Taylor is and I do think he's an idiot now. It really should say something when the developer of Unreal Tournament figures out that people play the singleplayer against bots more often than they play multiplayer, if at all. This applied to all versions of UT from the old UT to the new UT3. Ok, so Epic does doesn't polish or support their products or customers anymore, respectively, but at least they figured it out that even predominantly multiplayer games see a ton of singleplayer action. GPG should be paying attention here. That segment of the market cannot be ignored. Battlefield 2 has always been annoying to play online because of all the cheating, base raping, stat padding, low-ticket servers, and so on. I had my most fun with it playing the singleplayer 64 mod with unlocks 123 turned on.
None of the UTs have had a particularly engrossing story, and neither the UT or Battlefield series' have had strong single-player campaigns. Your examples are perfect examples of single-player "training grounds," so I don't know where your complaint really lies.
Don't get me wrong, a single-player component is really important to me. But if you're talking about a campaign and not just a sandbox or training ground, a single-player mode MUST train the player about the rules of the game, especially in strategy games where the emphasis is almost entirely on the gameplay.
I think people want an engrossing story to get them to play the game, but good single-player AI and multi-player keep people playing.