Dear Pedro,
it is not clear to me how a license that explicitly says it is okay not to contribute back would increase the numbers of contributions back to Roundcube.
From our perspective as a commercial entity, we have contributed quite a bit
of code for the 0.7 release, and might not have done that under a weaker license, as we prefer an even footing for all contributors.
So we would likely not agree to a less protective or balanced license.
That said, I don't think that is even a theoretical option. Roundcube incorporates some major third-party contributions under the GPL which are not easily relicensed, and obtaining permission from each and everyone one of the upstream contributors is likely an impossible undertaking.
You are of course welcome to re-write all those parts under a BSD license, but as a commercial entity we would probably hesitate to contribute to that effort.
Best regards, Georg