The same can be said for MCH's BtE and warrior's holmgang. The counter-play is not exclusive against summoner's burst or strong points.
And honestly, you'd be a damned fool if you're trying to hardcast something out in the open like that, and typically BLMs are higher on the interrupt list anyway because their stationary dps when given the chance is pretty absurd compared to a SMN's.
Speaking from a perspective of a MCH, this is not always the case or that straight forward. It's still a damage penalty if we're attacking from further than the sweetspot range even with GB, and its a signifcant loss in damage if it's closer than sweetspot. The other half of the damage is hitting with BtE, which can be finkity a times because your only reliable setup is bind, which lately has given me less-than-ideal potency boosts in my roulette.
Compared to a SMN's burst, it's not as potent, the effective cooldown is longer, effective range is lower and it's a bit more situational (especially with ricochet getting full potency), but it's faster.
The frantic and faster-pacing in seal rock naturally allows for the classes who are more motile and can still safely deal damage in large crowds to shine, and some are better than others in this regard. It might not be the case in wolf's den, but honestly wolves' den is a long ways away (3.2 last I remembered) and it's the only content that's still active for ranking up.
