I don't use any completion plugin with my Emacs. Not coincidentally I'm generally a better reference for our programs' APIs and organization than the documentation or the other developers who use things like PyCharm and Sublime.
For me not using completion works to keep me disciplined about the structure of code I work on. It also can make me vicious about arity and composability in code reviews. Fortunately most of those details are taken care of by design prior to implementation, so it's rarely an issue.
I think not using code completion might have more to do with you having an excellent memory and it might not be fantastic advice for everyone but I would be interested if there were stats on usage of features like code completion in emacs.
Its wholely unrealistic to suppose that someone would spend hours to days learning emacs but not spend 1 minute adding a completion plugin.