You can try the method you mentioned to see if that works. As of now, Siri is not programed to add contacts for you. It was never advertised to be able to do so...nor was it advertised to be able to complete all possible functions so it's not a bug or anything...it's just not in the programming for it to do so. You could always suggest it to Apple...they may add that in the future. No guarantee of course, but it doesn't hurt to suggest it. I actually had to enable siri to try it because I really don't use it.
Didn't mean to imply that you were having any unrealistic expectations...just was responding to you saying you thought Siri could do those things and that it can do appointments. And in general pointing out that it doesn't seem to be a bug or anything...just the way it's programmed for whatever reason.