Careful Nibbs, AMS lasts 5 sec (7 sec glyphed) with a 45 sec CD, Spell Deflection is passive and always available.
Slack, Spell Deflection wasn't particularly nerfed, the typical well-geared DK dropped from about 30% to parry to 20%, and the damage reduction was increased from 30% to 45%. It's the same coverage. And while you're right that you can't rely on it to cover EVERY major magic hit, possibly including one that might kill you (AMS is handy for that), it is a HUGE value when it triggers, and it is more than a little. Simple example, Frozen Blows. Hodir is going to hit you with five 20k frost damage hits every minute (actually a bit more than that, but we'll keep it simple). 20% parry chance means on average you'll catch one per chain (thanks to the random nature you could catch 2 or 3 in a row, though the chance is rather smaller). 45% reduction on a 20k frost hit is 9k damage knocked off the top. That's a very big save for your healers and not to be scoffed at.
It is situational, but the situations aren't so uncommon. Freya's Sunbeams, Mimiron's Plasma Blasts, Steelbreaker's Fusion Punch, Ignis's Flame Vents, just to name a few in Ulduar. You don't take it because it's a big always on deal, you take it because when you need it, it really can be a lifesaver. Though I understand that you don't like this kind of lifesaving when you can't press a button and know it will happen when you want it to.