Some banks may have continguency funds for when they are in financial trouble.
As someone has said, banks must pay out the big bonuses to attract the top traders, managers and executives that make the bank perform optimally.
So without bonuses (RBS board threatened to strike the other day at the thought of losing their bonuses) certainn employees would be obliged to move to other banks where there are larger bonuses, or, if bonuses are reduced dramatically, employees may strike.
They have high bargain power as they are very powerful people, who make huge amounts of money. Bonuses are therefore necessary and should not be saved (to an extent).
Even if bonuses were save, there would be high opportunity cost, as the money saved with annoy bankers, and would not even be nearly enough to bail out banks that have been effected by the recent financial crisis, which cost hundreds of billions of £s