the DR on defense affects Miss chance only, not crit reduction. That is not subject to DR.
539 is bad compared to 540 is because .04% of the time is enough of a chance to throw a monkey wrench into your healers rotations.
A crit is a LOT of extra damage. if a boss crits, you're almost gonna die guaranteed, especially with how hard bosses hit now. even having a .04% chance is terrible because you could be 4% away from a boss kill and suddenly that .04% RNG comes and thwomps you dead. Yes it is small, but as tanks, we are here to eliminate any possibility we can die by any means necessary, and this is a FOR SURE way to remove a double damage attack from ever coming.