If a project is releasing frequently and often, such as the X Windows system did in the early days, and people want the features in the new releases, the overhead in constantly forward porting private, proprietary enhancements (whether they be I18N support or driver support) tends to encourage people who might be tempted to keep code to themselves to contribute it back to the mainstream codebase. But if the project releases too slowly, or tries too hard to keep internal API backwards compatibility, it can cause people to fail to contribute their changes back, and then the open side of the project stagnates.
This is something that Charles Hannum, one of the NetBSD founders, eventually realized:
http://onlamp.com/bsd/2006/09/14/netbsd_future.html
In fact, he observed that a non-copyleft license tends to encourage fragmentation, and what do we see happening with Android? With a GPL license, things like Touchwiz would be have to be released as source code. (Of course, with a GPL license, there's a chance handset manufacturers would have been scared off, which is why apparently Andy Rubin very early chose Apache instead. Personally, I don't believe that to be true, given that Android did use the Linux kernel, which was GPL'ed. But, he was the startup founder, and he didn't want to take that chance with his baby, which is totally understandable.)