There's an important thing to remember when talking about the GPL: the impact, good or bad, on the practicalities of software development have nothing to do with its objectives. The authors of the GPL, the Free Software Foundation just aren't concerned by the kinds of issues that Daniel raises. The fundamental principle of the FSF is that anyone should have complete freedom to study, modify and share all computer software. The end-game of the FSF is that all restrictions that apply to software, such as copyrights and patents, be abolished altogether (in so far as they currently apply to software). In other words, they want any software written by anyone to have no legal protection from examination, modification and redistribution by anyone else. The FSF really wants all software to be in the public domain, without the option of copyright protection.
The GPL is their attempt to bring this about given the reality that copyright of software exists today and shows no signs of going away. The GPL is designed to use the copyright system against itself; the FSF calls this "copyleft". In typical commercial software, the copyright is used to prevent the licensee (user) of the software from doing things with it that the author doesn't like (such as modification or redistribution). The GPL instead prevents the licensee from adding any new restrictions. The end result of putting software under the GPL is basically the same as if copyright protection for that software didn't exist at all.
Now, you can make many arguments pointing out the practical downsides of removing copyright restrictions on software (whether by abolishing software copyright, or via the GPL). Daniel's post is a good example of this. But to the FSF, these considerations are secondary to the principle that any restrictions to what users can do with their software is morally Wrong. You can see an example of this in the GNU Manifesto where they acknowledge that adopting their policies could result in programmers earning less. Their view seems to be that doing the "right thing" can have effects that aren't necessarily desirable, but we just have to live with as best we can. Perhaps a good analogy is free speech, which necessarily increases the risk of children accessing pornographic or violent material.
I try to take a pragmatic "what best serves the common good?" approach to matters, which leads me to agree with the FSF with regard to software patents, but disagree with them on copyright. I think patents, when applied to software, are all harm and no benefits. For copyright, I can see greater upside than downside to the current system. It's true that without software copyrights we would all have greater freedom regarding how we can use our computers, but it's also likely that our computers would be of much less practical use to us. Do professional photographers and artists prefer GIMP or Photoshop?
Just to be clear, arguing over the implications of the GPL on software development practice is a very useful thing to do. Many people who don't care about the political position of the FSF still chose to use the GPL simply because it happens to suit their needs, and there's nothing wrong with that. But having a clear understanding of the costs and benefits is essential.