I'm a word geek, and I love Scrabble(tm). I play both online and with a Nintendo DS, sometimes against people, sometimes against computer opponents of varying skill levels. Since I began playing computer Scrabble, my scores in real life games have gone from averaging 280-ish to 325-ish.
This isn't a very scientific study as much as it is a look back through the scoring scratchpad that I keep in my Scrabble box.
I would hesitate to say that it's because of the video game; I feel it's more a matter of practice, practice, practice.
I have no doubt that video games can help someone learn or develop a skill. If it's presented in a real-world context with accurate feedback (e.g., first-person shooters won't give you gun-handling skills until there can be accurate kickback messing with your aim and you can't learn how to play a guitar from Guitar Hero), it becomes practice. Driving simulators, cockpit simulators...these are all examples of technology that aid in learning.
I am not familiar with Capitalism, and while you'd be able to understand some generalities about economics, many of the nuances would be lost. The variables that drive nuances are too numerous to program in a consumer level game. Heck, economic forecasting models that run on supercomputers still miss the mark.
As to learning language from an MMORP, my sense is that yes, you could. But it would be the same context as an English immersion environment. One caveat is that MMORP language is very limited in its vocabulary, and user comments are often in less formal/slang. You'd learn a subset of English, but not necessarily the one you want.
Overall, I think that video games have a place in the milieu of learning. Some people learn better with audio-visual/experiential tools than with books and paper. I think that if the game provides a relevant simulation with accurate feedback, it can be a valuable part of a comprehensive lesson plan, serving as practice for the real thing. Unfortunately, programming something accurately enough to provide that level of reality just isn't practical for consumer-level gaming.