Actually, what you think is free for the Express editions is only free for the first year. At least that is what some of the information stated as I was reading the various screens on my way to download the Express edition. If I recall correctly, the plan is to provide the express editions free for the first year, after that there is a charge, cant recall what they quoted as being the target price tag for the express editions, its not a lot in comparison to the other products though.
Putting in my two cents worth, and from a somewhat different viewpoint, the Express editions may be an avenue to provide initially a zero cost and later a lower cost way for those who program as a hobby or for those who program for Open Soruce projects. One of the big roadblocks for .NET programs making it into Open Soruce was the high cost of the IDE, compared to what other languages like Java and PHP were providing. If you are an avid PHP programmer, you could get from Zend the equivalent of the VS.NET Professional IDE for less than half the price and if you wanted to use something totally Open Source then Eclipse was right there waiting for you (at least I think it could be used for PHP).
Anyway, it was just a thought so figured I would share it as to why I think the Express editions exist.