This strikes me as a weird decision. Other than "so they have something to put in macOS 10.12" does anyone have any reasonable speculation as to why they didn't include all the bells and whistles in Messages on Mac? Seems stupid to have to leave the Mac to go to an iOS device to reply in kind to a message you've received. My own assumption is that it would require a rewrite of the Messages app on Mac from the ground up, and they either couldn't be bothered, or just didn't have time this time around to implement it. I'd like it come in a point release but I'm not holding my breath.
Who knows they simply didn't get it to work safely, comfortably, or up to their standards, and they need more time. Or they hate us and force us to use iMsg on mobile over desktop just to sell more mobile devices and move away from desktop solutions #tinfoil