starcraft-2
I don't have Heart of the Swarm, and I don't plan on purchasing it in the immediate future. However, I know that some of my Battle.net friends have gotten it. Will they still be able to play Wings of Liberty matches with me, dropping all of the changes made by the expansion?
Edit: If nothing else, I would love to be able to play custom games with my friends, even if we can't play in the leagues, since that's all I did with them when we played.
If I'm understanding your question properly, in order to have two Heart of the Swarm licenses you need two Wings of Liberty licenses.
Since Heart of the Swarm is not a stand alone expansion, you need to have all the prerequisite games in order to play it (each copy).
You can see for example, on this Cataclysm box art for World of Warcraft, that it requires "Wrath of the Lich King to play". Wotlk says it requires TBC, and TBC requires the base game. It is expansions all the way down. This trend stands true for all Blizzard games to date, as they don't release stand alone expansions.
I was wandering around Arqade Meta and found this:
This is currently the case for Starcraft 2 HOTS. Everybody can preorder on Amazon and get a free Beta key.
Best Answer
Also be aware that Blizzard enabled "spawning" a while ago, which will automatically bring all players up to the highest level of anyone in their party. So if you play in a group with someone who has HoTS, you will have access to the HoTS ladder and units.