It's slowing down the response times; most drivers round here don't seem to use their eyes and only get out of the way when ( if! ) the siren does eventually go on. It is certainly resulting in considerably more near-misses with the pedestrian texting zombies nearly stepping out in front of ambulances ( ironically ).
I wonder how those who complain about the sirens would feel if they themselves had to summon help and it took a minute or two longer to arrive because the vehicle was running on the lights only for most of the way?
On a similar note, the unmarked cars definitely do need sirens (and seem to always have them one when on business). I head the sirens a good amount of time before I saw a black BMW emerge from a side street with tiny flashing lights in it's grille when I was driving around the other day.