What
@lorenzo75 said. Let's say there is an "even fight" between two guilds, 10v10 players. If 4 people are trying to siege and 4 people are trying to place def the defending guild has the upper hand anyways, because there are several slots for defending while there is only one slot to siege. With your change defending is even easier, because it is quicker.
So the guild that sieges either
a) gives up in frustration
b) needs more goods (and luck) to get the sector, because.. well.., sieging costs goods, you have to confirm that*
c) has to wait until the other guild tries to attack so they can defend successfully, because... it's easier
Sorry, this is not good. Please revert the change.
*) By the way: when trying to siege quickly, I don't even look at the costs.