I have always thought that the prinicpal practical reason for the GPL (aside from the philosophical reasons of freedom) is that without the safeguards of having to release modified code, a company can use the vast library of GPL software to short-cut their development process and then make profits without having to financially recognise the contribution from the legions of people who have contributed. Like patents and copyright, the GPL is a bargain between contributors in addition to those provided by usual copyright protection - sure you can use this code, but contribute the results back into the pool, if you don't like it fine but develop your own.

This is why the Tivoisation problem is so difficult, in principal you have the software but you would also require to build new hardware for it run on - the new GPL is an attempt to deal with this but is in my opinion slightly misguided as GPL deals with software abstractions and it would be very difficult to make restrictions restriction on the hardware that abstraction executes on. Hardware is covered by different legislation anyway and no software developer can insist that the designs for the hardware are also released. However, the flipside is that Tivo other projects have gained enormously from the use of GNU/Linux code and should have an obligation back to the contibutors for that commercial advantage.