You Get a Swarm Host - You Get a Swarm Host - Everyone Gets a Swarm Host

StarCraft 2's "spawning" system forms a convenient analogy to Heart of the Swarm's zerg theme: a single player with the expansion can infect his party members with it, spreading the new units and unit modifictions to them (at least, for the duration of the party). Of course, this infection is highly beneficial to players who haven't purchased HotS, so, maybe the analogy isn't perfect.

It's a classy move, although it's hard for me to see it as generosity -- surely this is a sign that the expansion's uptake hasn't met Blizzard's expectations. But in that regard, it definitely is an effective form of in-game marketing, showing Wings of Liberty players what they're missing first-hand. And it's definitely better than the previous system, which forced players down to the lowest-common-denominator expansion level of their group (meaning HotS owners wouldn't actually get to play with the expansion content).