I don't see why that feature needed to change the normal + feature anyway. Couldn't they differentiate between it being at the start and in the middle? It'd be obvious for power users from the recommendations. That said I can see the argument for just supporting single word quotes and to me that seems more intuitive.
But it would be at the start in many cases. How would it know if you want to go to the Google+ site of YouTube or just search all the pages that contain the exact phrase "YouTube"?