I agree with Robert Treat's blog What the free non-free databases signal that in the end the commercial database vendors are going to loose out in the same manner web server vendors have lost out to Apache, lighttpd, tux, etc.
But I think there is another aspect of this that helps the Open Source database community more than anything. Providing these free installs tranishes all of the mystique of their products.
I remember when I was starting out as a programmer and had my first opportunity to work with a big name commercial database product. I felt I had finally arrived. I was one of the Big Dogs ™ now. Able to leap tall data structures in a single bound!
Little did I know that I would be quickly disappointed and spend the rest of my tenure at that company lobbying to replace the commercial package with PostgreSQL. Not only to reduce costs and improve performance, but to simply make my life/job easier.
If I had not been previously exposed to a product like PostgreSQL, I would have probably thought that insert commercial vendor here was the best thing available, based on advertising and hype. I probably would not even have investigated any further.
Having these free/non-free installations of the propular commercial databases will allow the next generation of geeks to evaluate both the commercial and truly Open on equal footing. And in this fair fight, we are sure to win!