In the context of Emacs, absolutely. I, for one, love the fact that the completions aren't an opinionated set of rules, and anyone can configure them to their liking. Why would I let anyone upstream dictate how those rules should apply, on my computer, in my editor? For example, when working in a Clojure codebase, I don't want to blindly rely on CIDER completions because they work only when you have a connected REPL. I want to leverage LSP completions (if it's connected), and I also want completions for my code snippets, filenames, keywords specific to the language, words in comments, etc.
If tomorrow I wanted to use different rules, applying them dynamically based on some conditions - Emacs provides ways to achieve that.
So yes, for completions specifically, I don't want them to be rigidly embedded in the core of Emacs. I prefer this feature to be an extension. Interestingly, Emacs is so versatile that some extensions are built so well that sometimes you completely forget they are not built-in features or an afterthought; they feel as if the editor was specifically designed to have them.
I have nothing against completions being configurable or extensible, but only about not having a reasonable default set of capabilities shipped with the system and having to depend on extensions of various qualities and states of completion and which may further depend on other 3rd party components to provide such a core functionality for modern work.
