I don't know why you need a great clock either, if you have stable, symmetric network paths from a central location to all your servers colocated at exchanges, you can predict the delay between sending from the server and getting to the exchange, you can split your order and send it to the various exchanges with appropriate delays and know that everything will arrive at the same time. If you're wrong, it's going to still be close enough that nobody will be able to see it on one exchange and react to it on another before your order gets there.

This was the first thing I thought of when hearing the flash boys story on the radio: The banker was complaining he couldn't capture the whole book across exchanges because resting orders were cancelled before his order got there -- he just needs to get his orders to arrive close enough in time (although expect a bigger tick, probably)