There’s more to WoW than just raiding. Also, there is a PVP talent as well as stackable Azerite trait that can significantly increase the healing of Light of Dawn. Additionally, it’s not just about worrying about overhealing so much as ensuring that Light of Dawn will transfer as much healing as possible to your Beacon. If you are in a situation where you will be one of the targets of Light of Dawn, this won’t happen if you’re sitting at full health when you don’t need to be. It boils down to managing your health as a resource.
In any situation that involves other healers, you don’t even have to spend mana because it’s almost a certainty that you will have HoTs rolling on you as well as AoE heals beneath your feet. In this scenario Light of the Martyr is damn near free instant-cast healing. Again, it’s about health management, even if the current low tuning of the spell makes the notion a chore not worth the effort.
I don’t view it as sacrificing health so much as a redistribution of damage. You could change the wording and adjust the function so that you heal the target and then 50% of the amount healed worth of damage will be redirected to you. While this would be more in line with how Aura and Blessing of Sacrifice work, it creates a potential for problems such as being bombed by multiple LotM targets getting hit at the same time. What we have now is just a lot cleaner, even if it requires a bit of forgiveness that the mechanic doesn’t -quite- match the concept.
At the end of the day, Holy Paladins are a heavily armored, shield bearing frontline healer. It just makes a hell of a lot of sense for such a spec to help bear the brunt of the damage so that his more fragile allies have less to suffer.
Similar to Spirit Link Totem, the sacrifice mechanics of HPal serve to redistribute damage to lessen the threat of death to any one member of the group and I feel like this concept should be given greater emphasis to help further define the role of the spec.