Sometimes a clean merge is possible but with a rebase, in-between commits may raise conflicts in which case a conflict must be resolved for each commit individually, which is not quite productive and at the end wouldn't add so much in how the resulting history looks like. With a "one-shot" rebase, a conflict (if any) is made based on the latest revision, then in-between commits approximated based on that resolution. This way the history can be roughly preserved with the same amount of effort while still using a rebase rather than merge.
