And many copyleft licences include terms that prevent any part of software covered by them from being included as part of a package that is not itself completely source-available. This reluctance translates into non-participation in many open projects by companies that sell their software. As a a consequence, they will not expose their customers to copylefted software, and thus an entire audience of potential users is missed. This allows the commercial sector to continue adding value to the product, and fuels the open project to re-engineer proprietary changes into the open version. If such proprietary enhancements are forbidden, on the other hand, the only origin for innovation is the open sourcerers themselves.
Competition feeds innovation and can add zest and interest.
I also tend to be in favour of dual licences that eventually lead to release of the source. End Notes 1 The OSI attempted to register the term 'Open Source' as a trademark, which would permit them to control who was permitted to use it to describe software. I don't recall if the application was rejected or if it was simply dropped, but use of the term is now essentially generic. If an enterprise patents a proprietary innovation and defends the patent, this positive effect can be negated.