I see the real benefit of the GPL being that companies like Microsoft can't incorporate OSS code into their operating system and then sell it on to clients all the while trumpeting their wonderful innovations and not disclosing that their wonderful innovation rides on the back of lots of OSS code.
If they want to tout that their proprietry model of software developement is the better one, let them do it without using OSS code. Since we don't have Windows code to compare it to, we can't know for sure if there isn't already any OSS code in there, but at least the law would be on our side should the situation ever arise where we do find OSS code in Windows.
GPL means that companies have to either support Open Source, or not. It ensures they can't pay it lip service and take what they need from it without actually offering anything in return.
What Sun is doing with Solaris and Java is an indication of the OSS world without the GPL. Sun gets to go on about how they support Open Source while controlling their contribution with an Iron first under a non GPL compatible license. There is not allot of freedom in their interpretation of OSS. The GPL wouldn't have allowed them to do that.