Politely giving them a few seconds of free shooting before you draw your guns is not a great survival strategy.
With the number of officers they often have in most cases it would make more sense to start off slowly and unarmed, making an earnest attempt to communicate with the target. People won't usually choose to fight a suicidal battle. Even if they're extremely upset and disagreeable almost everyone will go along with it if calmly presented with a warrant and given some time to think things through.
I do have the presumption that when professionals do things that seem weird, they probably have reasons that I as an amateur don't immediately understand.
I've also read enough Radley Balko to know cops often get away with doing awful and stupid things...
Unless it's proven someone is on imminent harm, then they should find another way to collect evidence, or just not do it.