"Great feature – thanks! This may be a stupid question. Never the less: Is this feature planned to be a part of Outlook for desktop as well?"
We live in an Online-First world now and some online features will never make it to the desktop. It should be interesting to see if and when it does appear in Outlook desktop. I'm picking that once Outlook modern app (Outlook Touch, Outlook Windows 8 Start Menu) is out, it will share the same code base as OWA and we may see features like this available both online and on-"desktop" (modern mode).
I discuss this question more in this post - What might Office Touch (Office Gemini) have to do with Office Online?