Yes, because you have to get hit 6 times in 20 seconds to get a full stack of the buff. As was mentioned in another thread about the Fervor/Oath, the following sequence is usually what happens:
1) You consciously decide to use the trinket. You get a 20-second buff that waits for you to get hit.
2) You get hit, take damage, and gain the first 1265 armor.
3) You probably dodge this one, or maybe it gets absorbed by shields. Nothing happens.
4) You get hit again, take slightly less damage. You gain the second 1265 armor buff.
5) You parry or the boss misses. Nothing happens.
6) You get hit again, take even less damage. You gain a third 1265 armor buff.
7) You get hit (zomg unlucky!), you probably get some damage shaved off from AD and gain a fourth stack of the buff.

You dodge again. The stack stays at 4.
9) If the boss has about a 2.0 swing timer, this is around where the buff falls off.
I'd rather have an on-use ability that doesn't rely on worst-case scenarios to be good... compare to the Black Heart, which gives you 7k armor like clockwork every 45 seconds.