The main purpuse of ESK was not to block users out from listening but rather to make it more difficult for someone to program a radio to key up on the system and generate false radio traffic. So it really was not a waste of time or money if it saves having emergency equipment responding code to a fake call.
That makes sense, but I think I just saw the "key" fly by a few seconds ago, and with a keyspace of only 255 possibilities it is pretty trivial to get on the system. It just means that to attack the system it takes a little bit more money to purchase the correct radio and an hour to brute force the key by hand--it doesn't really change the situation, just the hardware required for the attack.
Besides, an someone could always wait for an authorized user to key up and overpower the input signal to the repeater.
I guess if you look at the number of ESK capable radios in the hands of people that would want to cause problems it does buy some protection for a short while. It would make sense that they would change it at the last minute before the DNC without any public announcement--pretty genius actually, seeing as it really is the only way to make it effective for preventing tampering is to use the element of surprise while people scramble to get new hardware. It makes you wonder if they had a credible threat of someone using interference to disrupt their response during the DNC.