I've always considered multiplayer games as something beyond pure entertainment. My early experiences with text-based MUDs (precursors to highly graphical MMORPGs of today) showed that players spent significant amounts of time interacting with other players rather than just play the actual game itself.
While the moving-between-rooms-killing-hapless-virtual-beasts metaphor still remains, the more interesting applications of these virtual environments are starting to be appreciated at a level beyond entertainment for kids.
Note the statistic per The Economist, only 30% of gamers are under 18. The majority of ~40% are 18-49 and 19% are over 50. Most blockbuster games of today are targeted for the young and middle age adults.
I should mention that the Serious Games Summit coming up on Oct 31-Nov 1 in Washington DC will be discussing games as they pertain to Education, Government, Health, Corporate, Science and of course Military environments. The lessons to learn lie in Instructional theory and practice, Simulation, Contests, Public Affairs, Diplomacy, and marketing.
getting serious about "games" at the Serious Games Summit