Just based on what is known now, my opinion that revealing John Hurt as a mysterious forgotten Doctor made for a better cliffhanger setup for Day of the Doctor than having Paul McGann or Peter Davison or whoever would have. An unknown former Doctor, and he's John Hurt pretty much hooks the audience and makes them interested in the story which is going to be told to explain this. One of the actual former Doctors stirs up fan excitement, and I must confess I would likely experience a fangasm had the scene actually featured McGann, McCoy, or whoever. But once the nostalgia clears, what else is there to look forward to for the six months he have to wait?
Don't get me wrong, I am a bit disappointed Tennant's the only former Doctor returning, and no classic era Doctors. And I'm someone who considers Tennant to be "my Doctor." But John Hurt's Doctor might very well be a suitable substitute, and not the slap in the face some are already making it out to be.