I had this same thought, but I think it would just be best to use these sort of plugins wisely, and not on every form element possible. Or to maybe have some sort of form "note/tip" that mentions it's being auto saved. Sites like Quora do this via drafts of your comments/responses that you write. That way people know to erase the content if they truly don't want it saved.

Like many UX features similar to this, I would say it mainly comes down to who your audience is, and where it's being used on how "weirded out" users would feel by this feature.