I see two benefits that could be a real game changer.
The first is a more efficient development pipline (as described by AbstractKiller), that would lead to shorter development cycles across the whole Windows universe.
The second point is, that da Surface "Phone" will not be a "phone" that could be compared to an iPhone or Galaxy. It will be a mobile PC that can handle phone calls and creates a whole new category of devices that could realy attract consumers and could shake up everything.
You ask yourself: why?
1. Imagine a Surface "phone" (or lets call it "Surface Mobile") that has a formfactor like the X3 but can transform itself to a tablet and can dock to external devices (Display, Keyboard, mouse, Webcam etc. and perhaps external GPU) and transform to a "real" PC. In this scenrio consumers could ask themself if it makes sense to purchase a phone, a tablet, a Notebook and a PC when they could just purchase a Surface Mobile (and the benefits of having a "tablet everywhere" and no Need for syncing different devices.
Imagine the following scenarios:
1. You have a workdeskt in your office and your home Office where you can dock your SM and continue working without syncing etc.
2. You have a notebook-dock (like that for the X3) when you need a "real" PC on the go
3. You can dock your SM to your TV and it brings up the Xbox-UI that can be controlled with a xbox-controller that is connected to the dock (for Netflix, Webbrowsing, Music, showing your Pictures and lightwight gaming)
4. You can wreless doch you SM to a hotel TV and work while on the go
5. You can wireless project a "real" Desktop to every Windows Computer and use your PC on the go (for example with the PC from a collegue)
6. You can project a "real" desktop with the integrated (or attached) table top projector on the table and use your "real PC" everywhere
7. I dont know, but I think there are many more scenarios
So... yes... I'm still optimistic and I think if MS get the SM right, there will be a chance to create a while new device category that attracts businesses AND consumers