People always explain the existence of bonus culture through the firm's ability to manage comp downward in hard times. I'm not so sure though - I've been through hard times at a hedge fund. One year my bonus froze at the previous year's value, and another year the more senior people took a 10% hit to their bonus.

I honestly think bonus culture is about power - specifically the power you're describing there. Your boss's boss can ask you to come in at 2am because if you don't, you risk taking a massive hit to your comp.

The other reason - also related to what you're pointing out - is that if you quit six months into the year, you leave a significant chunk of money on the table. Silicon Valley has similar golden handcuffs in that regard though.