Many source code licences were approved as free software or open source by either the Free Software Foundation or the Open Source Initiative. The problem is that if one encounters a software package that is distributed under one of these licences, they will have to become aware of this licence’s peculiar restrictions. Furthermore, it will be difficult to tell whether it will be compatible with code under a different licence.
As a result, there’s the problem of licence proliferation.
Some licences are known to be incompatible with one another: the GPL version 2 with the GPL version 3 or LGPL version 3, the GPL version 2 with the Apache License, all versions of the GPL with the original BSD licence, and many other non-GPL-version-2 or non-GPL-version-3 compatible licences. As a result, often, a program that is open-source is still unusable, and will have to be reimplemented.
Furthermore, some projects or organisations may consider the software under Strong copyleft licences, or even weak copyleft licences to be too restrictive, and will instead opt to rewrite it. Here are some cases of open source code being made unusable due to problems with their licensing:
For example, the Free Software Foundation now started the GNU PDF project that is licensed under the GPL version 3, because all the other Free software PDF projects are GPL version 2 only. So because the GPL was used, the same problem need to be solved twice.
Another case where it happened was this story of an the Inkscape set operations patch:
Once before, someone had contributed a patch to add boolean operations, but that patch relied on a polygon clipping library provided under an incompatible license. There’s little more frustrating than having a solution in hand, only to be hamstrung by legal problems. Even though it was an important feature for us, we regretfully postponed development of it into the distant future on our roadmap and proceeded with other work.
Furthermore, the OpenBSD project are now re-implementing a lot of software that is only available as GPL or similar licences, under BSD-style licences, due to OpenBSD’s more pedantic licensing policy.
The GNU Project had to start working on develop the LGPLed GnuTLS because the licence of the also open-source OpenSSL library was not compatible with its GPLv2 and GPLv3 licences.
Reimplementing source code from scratch due to licence incompatibility is unfortunate, because open source developers have much more productive tasks to accomplish in their precious time. As a result, several open-source developers have opinionated that one should use a simple, non-copyleft and GPL-compatible licence, such as the MIT/X11 licence for all original source code, to encourage its reusability.
In an O’Reilly Media interview with him, back in 2005, Eric Raymond (who wrote the “Cathedral and the Bazaar” and spearheaded the Open Source Initiative organisation) has voiced the opinion that “We don’t need the GPL any more” and that people should avoid using that licence for new projects. Naturally, some people still disagree.