This video started a train of thought. I think I read the following in a magazine called "The Week" some years back. But, am not sure so I may be off on some details. Since, it is quite old try to read it in the "old times" context, not in the current political scenario.
Once a General of some country visited America. The American General showed him around their facilities and even showed him around their research base somewhere in Nevada. The other General asks the American General - "Don't you consider this a security threat. That is me seeing all this". The American General replies "Our openness is our strength".
Open source is no different. "We have nothing to hide" its open. This is how it is. A friend of mine once pointed out - then why do they hide behind the GPL (LGPL, APL etc for that matter... makes you wonder about his intent). Well, there is foolishness and there is strength. Giving away research (and weapons) as charity will eventually lead to those being used against yourself. Without GPL someone can just borrow some open source code make some small changes and claim it to be his own work (and "close source" it). The person who creates it tosses and turns in bed because, now, he is excluded from the growth of his own creation.
The foundation of open source is to bring together honest interest and participation in creating something great. More over, one may not be free enough to show off one's skills on a payroll. There are business models around open source, but the motivation may just be a reward unmatched by any other - Fun (2).
Watch this video. The comments on YouTube are interesting too. Especially the analogy of the cake recipe does a good job.
Is GPL enforceable? Yes, it is. There have been some interesting precedents. I learn't of this during a presentation by Herald Welte in Linux Bangalore. There are couple of interesting cases you can read about at GNU Monks and GPL Violations.