||ESR: "We Don't Need the GPL Anymore"|
The fact being that if the GPL were not really important to the goals of open-source software, then why does Microsoft find it such a horrible thing that its executives decry it as "a cancer" and other perjorative terms? I never hear this about the BSD license or other open source licenses, so there must be some reason Microsoft finds this license so distasteful.
The simple fact of the matter is the GPL prohibits a third party from making any product using that license into a proprietary one. None of the other licenses out there give the kind of protections against misuse of the ideals of open source, that if the author of the work chooses to share it with others, other people do not obtain any privelege to then take that same material and make it proprietary simply by adding something to it.
The GPL doesn't place any obligations upon a user of a product, or on a private modification, nor does it do anything to restrict others in the development of a competing product. It only imposes a restriction upon those who want to make modifications and release the product to the public. They cannot make those modifications proprietary and not allow everyone else to see them.
If we did not have the GPL or some equivalent license requiring that modifications of publicly released changes also be public, many of the advantages in functionality would have been "stolen" by proprietary software applications that no one else would have access to except the developers of the changes, and they could use those changes to lock people in. The inability to force proprietary lock-in is one of the strengths of the GPL and the reason that a company like Microsoft is deathly afraid of it.