I discovered this by accident some time ago as well but I've always appreciated the convenience - when I add an event using the Quick Add dialog, it's very useful for me to be able to say "Movie Night with alice@example.com" and have it show up on Alice's calendar automatically. I use it all the time at work and at home and it saves me time from having to go to the dedicated event creation page to add people.
I don't think anyone is arguing the potential usefulness of the feature. It'd just be nice to know when its going to happen and have an alert box verifying that you actually wanted to add the user to the event. The alert box could even have an "Always do this" option.
