Well, assuming that you are adding features in parallel you have an history. You have a branch with feature A and are about to build feature B, which is now based on A, someone else is building feature C which is also based on A. Building B and C might require different or equal changes to A.

There's no guarantee that any of these features will be built in order since they might have different priorities, difficulties or level of acceptance so it's hard to tell which one must or will be done in what order. Rebasing pretty much settles that while merging is much more sane to that workflow. It's harder to keep it functional yes, but it's enabling parallel development. Using rebasing and/or merge isn't a source control problem but a feature management problem.