It forces global choices to matter.
Right now as a Guardian druid, we have only a very small number of choices we have to make as we tank and that usually comes down to things like
- Maul over Ironfur? - Based on content difficulty
- High Damage so do you use Barkskin, Survival Instincts, or FR?
Because FR is on the GCD, it adds another depth to the choice you make for (2).
If the high damage is something you anticipated and something you just want to smooth, then you’ll make the choice to weave in something that is bound by the GCD, just like any other rotation ability. People want to view FR like its a defensive choice, but its more a rotation ability we weave in as needed than anything else, hence why its on the GCD.
If the high damage is something you likely didn’t anticipate and you need to minimize damage intake substantially, you use your major cds (BS/SI) which aren’t bound by the GCD allowing you to react to the unexpected.
The designers wanted to alter when we use FR this expansion, trying to get us to use it as a smoothing mechanism to our damage in-take in a way that it acts like barkskin but on a much shorter cooldown. It was never meant to be a reactive game play concept like it was in Legion, something that I think a lot of Guardian druids have yet to truly grasp.
I don’t think FR belongs on the GCD after 1.5 years of playing with the spec in this particular style, but whether its on or off the GCD really has minimal to no major impact on the specs strength. That however, very well may change with the potency changes to FR in 8.2 when consuming more rage for more healing.