I'm unfamiliar with the movement healing - will have to research that.
There isn't a whole lot of benefit for the Druid tank besides that they are pretty sturdy but still take chunky damage.
They need their defensive ability rolling (Iron Fur) and it settles in, but prior to that they can get spiked for damage. Their rotation is also very simplistic.

The reason they are used in our comps is to just make it 5x Druids and much more simplistic for other side advantages such as the entire team can then go cat form and stealth/skip certain packs of mobs. Hypothetically it also provides a brez, but it's risky since your tank would need to shift out of bear to get it off.
If it wasn't for the sake of having 5x of the same class. I wouldn't be on Guardian Druid. Instead I'd go.. I'm not sure but not druid