User blog comment:Blackendedsoul/Why not Shiki?/@comment-3297113-20110322225257/@comment-3242083-20110323003155

This.

The war against Whitebeard was planned to the last bit, and the location was one of the reasons why the Marines could win with relative ease.


 * Aokiji and Akainu's combo attack (that took out a huge amount of pirates). Extra devastating since the WB's had to bunge up that much.


 * The encircling walls and connected cannons, not possible to use at sea.


 * The army of Pacifistas could progress without trouble (bar Hancock).


 * The shichibukai could be gathered in one place. It is surprising that they all agreed on meeting at marinefold in the first place, can you imagine them going actively after WB?

All that said, Shiki was already the "big catch". Ace, however, could be used to lure the most powerful pirate into a situation where victory was actually possible. That the son of Roger would be killed in the onslaught, broadcasted to the whole world, is just hitting 2 birds with one stone.