At least Outlook uses this request when accepting an event invitation.
https://msdn.microsoft.com/en-us/library/office/hh428685(v=exchg.140).aspx
https://msdn.microsoft.com/en-us/library/ee158682(v=exchg.80).aspx
As I understand MeetingRequest is already part of T2319, but it looks that MeetingResponse could be implemented separately. Not that it's a simple task ;) We'd need to:
SendResponse element was added in version >=16.0 of the ActiveSync protocol, which means it is optional and can be excluded from the initial implementation. Also, from what I see it is about calendar events. I didn't found any information about handling of iTips for TODO (tasks), so they are probably ignored.
So, I started working on this. I thought it's reasonably simple to implement, but testing this with Outlook 2013 I see a lot of strange comminication that is not described in the documentation. It sends redundant Sync commands with event updates, so in the end, after accepting an invitation the event ends up being still in un-responded state.