I think it would be good to enable the autocompletion for @username also for comments which don't need a ping, e.g. first comment to a Q/A or if only the OP and one more user is involved in the conversation.
This would make it easier to introduce new users to the concept of using @username to direct a comment to a specific user.
Imagine this fairly common situation:
- new user ask a question
- I leave a first comment asking for clarification - without @username because the OP is automatically notified and I'm too lazy to type the name by hand or it contains special characters I cannot type.
- OP replies (of course also without @blub)
- I get notified and can reply in a timely fashion
- [optional: repeat this n-times]
- another user joins the discussion
- OP replies to me and I don't get notified and in the best case see their comment by chance some time later
- now I have to explain to the OP that they need to use @blub otherwise I won't see their reply -> this is hard to understand for the OP, because it worked at first
=> I propose to (optionally) enable the auto-completion for all comments, this way I could address the OP with @username right from the beginning and the OP would easily know that @pings exist.
Of course I could greet the new user by explaining the ping system in my first comment, but I think this would confuse the user even more because they would wonder why it is not working until a third user gets involved in the comments.