I feel that the short-answer is that the community of users would cease to thrive as many would look to other free alternatives. WordPress.com, their free public blogging site, would lose countless users to services like blogger, livejournal, etc. Beyond that, however, there are lots of other unseen factors.
Others have already sounded off in the comments (myself incuded) pointing out things such as the inherent value of something you have paid for verses something you have gotten for free, a very valid point. The flip side to this, however, is that by gaining the CMS for free the overall cost of developing a website is greatly reduced. As a web designer and developer, I am able to pass these savings directly to my clients and provide them a level of service that they otherwise could never have afford.
So, there you have it. What do the rest of you think?