Without this mythic, blocking always costs stamina, except when you're using a frost staff with that one particular passive.
I think the Stormweaver's Cavort might see more use if one of the other weapon types had an equivalent passive that made blocking with it always cost stamina, no matter what.
One hand and shield is the most obvious candidate, and there might be some potential in making blocking with the resto staff always cost stamina.
Also, as a side note... I wish the devs could revisit some of the least used sets in the game, and give them a small rework.