Once you miss a hotfix done on a release branch in your master you'll see the value of back merging release branches.

Long lived feature branches are sometimes inevitable.

Moreover, this model has no provision for code review as done e.g. by GitHub or GitLab.

In GitLab EE you can use a merge request to review code and then rebase it, see http://doc.gitlab.com/ee/workflow/rebase_before_merge.html

I documented an alternative model in http://doc.gitlab.com/ee/workflow/gitlab_flow.html

Also for large scale changes rebasing can be very difficult. Yes you can minimise it by rebasing often, but once you didn't manage to do it a merge can be easier.