Crews may indeed ignore an RA
if the situation calls for it, such as the scenarios described in this thread.
From FAA
AC 120-55C:
(2) When an RA occurs, the PF should respond immediately by directing attention to RA displays and maneuver as indicated, unless doing so would jeopardize the safe operation of the flight or the flightcrew can ensure separation with the help of definitive visual acquisition of the aircraft causing the RA.
By not responding to an RA, the flightcrew effectively takes responsibility for achieving safe separation.
Some TCAS horror stories from the past, compiled from NASA reports:
TCASII--Genie Out of the Bottle?