I think this has some drawbacks because it seems to require a lot of discipline from all the parties, to keep the source code clean once you are ready to merge and even if you have some presubmit checks if a human makes a mistake and has some typo in the comment, that might end up in the main branch. Ideally all the new changes are small but that's not always possible and the larger the change the higher the probability to leave comments around. I agree it's not the end of the world but such things do add up over time and if you are in a big shop things will definitely get messy, it's just a matter of time.