I can only imagine that with the implementation of a bunch of OT skills across all jobs (Buffing of TBN on others, Nascent Flash, Heart of Stone + Aurora) mitigating PLD being the de-facto OT, that they didn't want this skill to be a reason to prefer PLD in the MT position.

Since, as you say, it's free damage. But only when you're blocking. Which means it's DPS advantageous to be MT as PLD (Sure, there are AoE's that you can block as OT, but only as MT will you have it up basically on CD)

Which might not be something they wanted to do. Given they seemed to care enough to not go through with their "MT vs OT" balance style and instead opted to make all tanks be able to perform reasonably well in both subroles.

Of course, they could have always just reworked the skill to not be proc'd from blocking and simply be an oGCD skill to use with possibly an increased CD... But *Shrug* who knows what's going on in SE's minds, where skills like Shield Swipe, Steel Peak, Howling Fist and Dark Passenger get axed. While skills like Shield Bash, Mantra, Fire II, Blizzard II and Living Dead remain untouched, being neither removed or reworked into something better designed...