Yes, I know Apple has filed patents for Siri for OS X a long time ago and they did not include the voice assistant in the operating system. It’s the way Apple does this. They do not show half-baked features (ok, at least not too often).
Microsoft is apparently getting ahead of Apple on this and they will include Cortana in the next version of Windows 10. This is just an alpha version of the Voice Assistant, but it looks really nice.
One could only imagine what you can do by having this on the Mac. I personally use Siri a few times a day on my phone and I’d rather type on the Mac using Alfred to find a document or launch an app, but Siri can be useful in a hands-free scenario.
Also, Windows 10 is not due till summer or fall of 2015 (if Microsoft will deliver on that) so we may see Siri on OS X 10.11.
Take a look at Cortana for Windows 10 and let me know what would be you favorite things to do with it or Siri.