I don't know about tmux, but personally I find that it's pretty rare that you want to apply a submitted patch verbatim.
Generally there's a need to clean it up a bit and bang on it with some testing. I suppose that bigger projects can just bounce that back to the submitter to fix up and resubmit, but if a smaller project wants to encourage contributions the maintainers really need to do that.
There's also the point that projects with established mailing lists where patches are discussed probably don't want to move to github's web UI for those discussions.
Generally there's a need to clean it up a bit and bang on it with some testing. I suppose that bigger projects can just bounce that back to the submitter to fix up and resubmit, but if a smaller project wants to encourage contributions the maintainers really need to do that.
There's also the point that projects with established mailing lists where patches are discussed probably don't want to move to github's web UI for those discussions.