One use case I imagined was that you can rebase freely, and people who've pulled will have everything just work assuming they always pull rebase. But I may have been too pessimistic. A normal pull rebase may usually just cope with the sort of rebasing people are likely to have done upstream anyway.
The other question is, are you losing old history by rebasing older commits? Well, I don't suggest doing it for very old commits, but I guess, you're not losing any history for commits that were in releases.
Although that itself raises a reason to have a connection between the new branch and the old: you shouldn't be rebasing history prior to a release much (usually not at all. maybe to squash a commit to make git bisect work?) But if you do, you don't want too parallel branches with the same commit, you want to be able to see where the release was on the "good" history (assuming there's a commit which is file-identical to the original release commit), and fall back to the "original" history only if there's some problem.
And as I've said before, another thing I like is the idea that if you're rebasing, you don't have a command that says "do this whole magic thing in one step", you have a thing that says "construct a new branch one commit at a time from this existing branch, stopping when there's a problem", and there is no state needed to continue after resolving a problem, you just re-run the command on the partially-constructed new branch. And then can choose to throw away the old branch to tidy up, but that's not an inherent part of the commadn.
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