Light of Dawn doesn't heal for a lot- but it's an instant cast, frontal-cone AOE heal. It's something that you can hit the button and it happens. It requires skill similar to the use of Shockwave, the warrior tank frontal-cone-AOE stun. You never will get a perfect use of it- but the question is, can you get it so that it's better than what else you can use? In a 5 man, for instance, you don't need to hit all 4 other people with it- if you can get the tank and maybe one of the DPS in the hit box, if there's AOE going out, that should be enough to keep them alive- all you've committed to it is a GCD.

It can also be used while moving- moving to a place where you can hit them with another one later- or moving out of a fire, or moving to avoid lasers.

Theoretically, if you have AOE damage going out, let's say you need to heal all 5 group members, you need to move, and can only reach 2 people with Light of Dawn- I think it'd be worth it to use it anyway; you heal 2 people plus yourself, three in all, then holy shock-word of glory to keep them from dying...

And while doing this, you can have moved across the room, and still have kept people from dying. Admittedly it wouldn't be optimal, but it's more mobility than paladins have had before... and I think that's the big niche that Light of Dawn helps fill- mobility and mobility AOE healing.