Will Macs with macOS gain voice activated ‘Hey Siri’ functionality?
Indeed some future and perhaps some past versions of Apple Macs will gain access to the current ‘Hey Siri’ voice activated functionality seen on some adequately equipped iOS devices.
Hey Siri, Can You Hear Me Now
Apple does not currently have a far field microphone array  on any of its Mac systems. One of the reasons Amazon’s Echo/Alexa is so powerful is the six radial and one axial far field microphone array. Although the usual distance from the microphone to where you are positioned is not entirely a far field distance, a good series of far field microphones would ensure the best reaction time and discrimination from noise sources.
Apple is not using advance noise and echo filtering software or hardware currently on Macs or macOS. Again Amazon’s Echo/Alexa uses advanced hardware and software voice and echo filtering and centering technology. In fact iOS has far more complex hardware and software centering around the A9 and M9 processors. It is important to note that there are a number of technologies Apple uses on iOS. Most of the audio technology is to filter out ambient noise for optimal voice conversations. These optimizations do not fully align with voice recognition optimizations.
Apple also uses many aspects of the M9 processor to mediate the ‘Hey Siri’ response system. Not only does this conserve power, it also has the electronic to quickly detect that action word and to begin to record a speech snippet to send to the cloud for domain and intent extraction.
Dana Massie is an audio scientist (SoC Audio Architect) formerly from Audience, a chip technology company Apple used for easy versions of Siri. His work has been used extensively in the M9 processor and will likely be a big part of Apple’s Voice First systems along with new versions of Mac systems with Siri optimized far field microphones, filtering software and hardware.
Apple has solved the problem of continuity and preferred device selection if there are more than two devices that can respond to ‘Hey Sir’ and will be integrated in iOS 10 and macOS Sierra.
Siri on macOS is also a very early work in progress. Apple currently sees fundamentally different primary uses cases for Siri on a computer versus an iOS device. Thus even thought Siri can and may soon be, always-on by the release of macOS Sierra it may be constrained to a different focus.
It is my view that Apple will release a Macbook Pro with a version of the M9 processor to mediate Siri sometime this year. There will be other enhancements that may either eliminate cooling fans or significantly reduce their noise. I also feel certain Apple will release a Voice First system in a similar concept to Amazon’s Echo but with a few interesting advances.
Thus it is a 100% certainty that at some point ‘Hey Siri’ will work on some newer versions on Mac computers as when as Apple’s Voice First stand-alone systems.
 Understanding Microphone Sensitivity (http://www.analog.com/library/analogDialogue/archives/46–05/understanding_microphone_sensitivity.html)