There's a tension: the great advantage Siri has is a constrained domain (as you note); adding patterns, services and "ways to add things" etc expands this domain. (And adding new patterns, if they amount to grammar rules, can increase the domain dramatically.) It's a tradeoff between expressiveness and error.

My information is that the progress in voice recognition has been terribly slow, and in the last 10 years or so has been made mostly by limiting domains.