This would be very useful if they replace Windows 10 Mobile with a variant of Windows 10 for ARM (with Win32 on ARM) with a more flexible UI. That would be better than current Continuum, dock it and not only does the UI change, but this time you're running full Windows capable of running Win32 software. THAT would be the target goal for a Surface Phone, IMHO. Intel certainly doesn't have anything competitive with SD 835 in that power envelope.
Anyway, in order to make full Windows power efficient enough for a phone, it needs aggressive throttling of background tasks. This will benefit other mobile devices too (tablets, ultrabooks, etc) but those devices have been doing well enough without this. For an all-day phone this is an absolute must, which is what got me thinking about full Windows on ARM phones in the first place. It's like a bunch of puzzle pieces, Continuum, OneCore, Qualcomm/MS Win32 on ARM partnership, Power Throttling, Adaptive UI. I feel like they're starting to come together. We'll see though, the concept is easy but execution is tougher.