The decision between the GPL or LGPL for a library is a complex one, particularly when that library solves a new problem or an old problem in a new way. TrollTech faced this decision for the Qt library, and Nokia (who acquired Trolltech last year) has now reconsidered the question and come to a different conclusion. Having followed this situation since even before Qt was GPL'd, I was glad that we have successfully encouraged the reconsideration of this decision.
Years ago, RMS wrote what many consider the definitive essay on this subject, entitled Why you shouldn't use the Lesser GPL for your next library. A few times a year, I find myself rereading that essay because I believe it puts forward some good points to think about when making this decision.
Nevertheless, there is a strong case for the LGPL in many situations. Sometimes, pure copyleft negatively impacts the goal of maximal software freedom. The canonical example, of course, is the GNU C Library (which was probably the first program ever LGPL'd).
Glibc was LGPL'd, in part, because it was unlikely at the time that anyone would adopt a fully FaiF (Free as in Freedom) operating system that didn't allow any proprietary applications. Almost every program on a Unix-like system combines with the C library, and if it were GPL'd, all applications would be covered by the GPL. Users of the system would have freedom, but encouraging the switch would be painful because they'd have to give up all proprietary software all at once.
The GNU authors knew that there would be proprietary software for quite some time, as our community slowly replaced each application with freedom-respecting implementations. In the meantime, better that proprietary software users have a FaiF C library and a FaiF operating system to use (even with proprietary applications) while work continued.
We now face a similar situation in the mobile device space. Most mobile devices used today are locked down, top to bottom. It makes sense to implement the approach we know works from our two decades of experience — liberate the operating system first and the applications will slowly follow.
This argument informs the decision about Qt's licensing. Qt and its derivatives are widely used as graphics toolkits in mobile devices. Until now, Qt was licensed under GPL (and before that various semi-Free licenses). Not only did the GPL create a “best is the enemy of the good” situation, but those companies that rejected the GPL could simply license a proprietary copy from TrollTech, which further ghettoized the GPL'd versions. All that is now changing.
Beyond encouraging FaiF mobile operating systems, this change to LGPL yields an important side benefit. While the proprietary relicensing business is a common and legitimate business model to fund further development, it also has some negative social side effects. The codebase often lives in a silo, discouraging contributions from those who don't receive funding from the company who controls the canonical upstream.
A change to LGPL sends a loud and clear message — the proprietary relicensing business for Qt is over. Developers who have previously rejected Qt because it was not community-developed might want to reconsider that position in light of this news. We don't know yet how the new Qt community will be structured, but it's now clear that Nokia, Qt's new copyright holder, no longer has a vested interest in proprietary relicensing. The opportunity for a true software freedom community around Qt's code base has maximum potential at this moment. A GUI programmer I am not; but I hope those who are will take a look and see how to create the software freedom development community that Qt needs.