Block with or without Glancing Blows?

What people tend to forget is that for many builds its very hard to reach a significant amount of block without much investment e.g. extra items, extra passives.
Some builds thrive on max block that is linked to other perks like life/ES on block or whatever. But for a "casual" build that happens to get to 30ish % block and you are near the GB node anyway, its way better to take it and dont invest much into block elsewhere but rather some life nodes. Calculating whether GB is better or not also depends on how much investment do you need to take elsewhere to reach max block? Instead, you could get GB and some life or damage nodes.
What's killing you in the game is primarily single, heavy hits. Its better to avoid these more often but instead of avoiding it completely, you get hit for 35% of the damage which in most cases wont kill you.
I think GB is of great value for builds that want to significantly increase their survivability without much investment IF they by default reach 30ish % block.

TL;DR: Either invest into max block or take GB if you can reach 60-70% block with it w/o investing much into block elsewhere.