If the cops actually had anything real on him, they could get the warrant over the phone while stalling you at the door, or even storm your place and get it later.
And even if they don't get it, no cop can get in trouble for the raid if they "suspect" you might destroy evidence, and anything they find can still be used in court.
It completely invalidates your 4th amendment rights, but congress felt this was needed to protect you against "terrorists" 25 years ago.