The highest damage use of a limit break is to AoE a trash pull. If these potencies are still current, and there are at least four targets, a caster LB1 will outdamage a melee LB2. If there are seven targets, caster LB1 outdamages melee LB3.
Therefore, a ranged DD should BigShot/Skyshard the largest pack of mobs in the dungeon if you want to use the LB to greatest effect.
On the subject of using a healer LB to gain more nuking time, it would only be worth it if you could manage to do 5000 potency of damage in the time bought by the LB. This would require two Holies/Gravities on about 16 targets, or three Holies/Gravities on about 10 targets. As the LB1 only restores 25% HP, I have trouble believing that it could buy you enough time to cast three or four more Holies/Gravities in excess of the ones you can already cast with Swiftcast and stuns. It would certainly not be worth it to healer LB to buy nuke time if you're casting anything other than Holy/Gravity.
Having a BRD/MCH using FoeRequiem/Hypercharge to boost your magic damage would make it easier for you to reach those numbers, of course, but in that case, the BRD/MCH should be the one LBing anyway, not the healer.