Excellent answer acraig, as usual. :-)
There is another reason that in some ways is the most critical of all, along with #5 on acraig\'s list. Rapid releases are by far the preferred way to deliver any significant development project. Pro games would do this too but rely so much on secrecy that they cannot really do it. Rapid or interim releases have several benefits:
a) Testing by the public is much tougher than testing by devs, because of all the h/w permutations and \"dumb\" users doing things we never thought of.
b) The general public testing means the server is stressed much more than if a private test is done. We are close to 100,000 player accounts now, and have generated > 1 million crystals in MB.
c) Giving people the opportunity to hack and cheat here early lets us detect and catch their exploits earlier when there is less impact.
d) Finding all these flaws now means there is less lag time between when a feature is implemented (and bug caused) and when it is detected. With the revolving door of Open Source developers on this or any project, this shortened time means that the dev who made the error is more likely to still be around than if we waited 2 or 3 more years for the final product.
Hope that helps.
-Venge