The 20s cooldown on mantles make it so if you were to swap weapons to heal in a lrs/raids, you get left very vulnerable for a long duration of time. This means you are useless for 20s as engaging in a fight is not worth the risk of instantly dying.
(the same could be said for assassin's clones)
- another method is to allow mantles to stay for 5 seconds after swapping weapon (leaves enough time to drink a heal pot and swap back)
I dont wanna be the guy that says "skill issue" but if you're low hp you could check if mantle is off cooldown, and if it is, you switch to your rally weapon first and then cast mantle