I'd rather spend time writing new branches against approved, merged upstream rather than submitting to an upstream branch that may never get reviewed and merged. Keep in mind, too, that the longer a branch lives the more rebasing it goes through. Also, the bigger the branch is, the more it is to review when finally considering whether to merge it. I think giving people a foundation on which to build is a good way to prevent a lot of extraneous work and also to build confidence in the direction of development.