While this is probably a feature some users appreciate, it surely isn't everybody. Doubly so if you use a desktop app as well, where the feature is disabled and you suddenly don't see mails you know are there- since they are now sorted in somewhere else and don't show on top.
This completely breaks the UX for me and I will change clients if this can't be disabled. As I said, I understand that users find it useful, I like it to be an option on the folder like in TB desktop. I do not claim the feature should be removed.