You've lost surprise - and the psychological shock that comes with it. Remember that the Allies wanted to maximize that shock, because it gave the dissident forces inside of the Japanese cabinet the most room to challenge the continue prosecution of the war.

In this, they were absolutely correct.

Possibly, but it seems speculative to me that this was an important thing. I imagine it would have been very surprising and shocking to have a demo nuke dropped a few miles away from Tokyo. Maybe less shocking, and that difference in shock value reduces the probability of surrender by an unacceptable amount?