As a jujutsu user, I don't disagree. I can see the appeal of doing a megamerge, but routinely working on the megamerged version and then moving the commits to the appropriate branch would be the exception, not the norm.

I gather one scenario is: You do a megamerge and run all your tests to make sure new stuff in one branch isn't breaking new stuff in another branch. If it does fail, you do your debug and make your fix and then squash the fix to the appropriate branch.