Yes what you are getting is potentially a much more fulfilled single player experience, yet as of this time if you are serious about multi player you will have to buy each expansion and because Blizzard now requires their permission first before a SC2 tournament is held and the lack of lan the user will have to use their own account which they have all expansion to be able to compete in the tournament.How will the expansion sets impact multiplayer gameplay?
The expansion sets will add new content to each race for use in multiplayer matches. This could include additions such as new units, abilities, and structures, along with new maps and Battle.net updates.
If I buy StarCraft II but don't buy any of the expansion sets, will I still be able to play online?
Yes. This will work similarly to Warcraft III and the original StarCraft, which maintained separate online gaming lobbies and ladders for expansion set players and players with the base Warcraft III or StarCraft.