To add to Xain's post:
Yes, the first release was a multiplayer <i>compatibility</i> test -- it was designed to ferret out any problems with different hardware configurations that could crop up so that the developers could make sure the final product was as compatible as possible.
The second release was also a test, and while it was also a refined compatibility test, it was also tweaked to improve gameplay. Those who download the test are generally experienced, hardcore players, so they know their stuff -- if there are any problems with gameplay, these players are going to find them. The developers wanted to see how gamers used the classes and their abilities and wanted to make sure that any problems in the gameplay were ironed out.
This release today is an actual demo, not a test, and is designed to give those people who didn't buy the game a chance to try it out and see if they'd like to get it.
- HC