This is a good discussion.
I'm not saying that I would macro HS simply because it's not on GCD and I can. I would macro it because of the relative benefit in mitigation compared to it's uptime and cooldown. In the current healind model where damage is less bursty and overall damage taken has value, having an automatic +20% block value every 10 out of 30 seconds sounds pretty good.
Obviously if it was a fight where I knew the boss was able to hit a lot harder for a brief period of time or I would sit on it for a few second and wait for that moment. But I wonder what kind of handling of this CD would make it more optimal than just having it up 1/3 of the time. If there are any spreadsheets or logs to show I'm dead wrong, I'd be interested in seeing them.
While there are probably no logs as of yet on the differences between situational/on CD use vs. just on CD use. The fact that paladins are given a fairly significant CD on short cooldown is very much paralleled to warriors. The similarities between the abilities is too uncanny and it's safe to assume they should be used in the same manner (Both short CDs with damage reduction through blocking).
Warriors consciously decide when and how to use their SB, and it should be no different for paladins. You will be making the choice on whether or not to use it based on a couple things.
1. Is there anything coming up in this encounter that will reduce the effectiveness of this HS (e.g. Atramedes flight phase/tank swaps)? No = Use it, Yes = Save it
2. Would this be better used (or be needed desperately) within the next 20 seconds of downtime of HS? No = Use HS, Yes = Save it
This is exactly why I don't mind the change to HS, while it is very similar to SB, homogenization and all that, I don't mind having to put more thought into tanking. Paladins were kind of lacking any real thought behind their own survivability.
Well, yes I will concede that it won't be a good idea to use it at times when I'm 100% I'm not going to take any melee damage. That doesn't say much though about the ability adding decision making to our tanking.
And it shouldn't be compared to warriors sheild block. Their ability has a 1 min CD and not 30 seconds. That changes a lot. It not only increases the penalty of on CD use of 20 seconds downtime to 50 second downtime, but it is now an ability that might be more optimized by lining it use up with forseeable damage intervals rather than just using it whenever you can.
Since HS has such a large uptime/downtime ration and its benefit is difficult to measure (if you're not block-capped, it has a chance of providing no additional damage reduction). I still feel it might be better to use whenever you can and let the mitigation it may provide as added relief off the healers.