Bungie hasn't exactly commented on exactly what Radiance entails, but I don't think that they're really a healing sub-class in the strictest sense of the archetype. Healing can be tricky to balance in games, if it's too dominant then the healer becomes an essential part of the metagame and it beats out other choices due to the importance of the role. If it's not dominant then it gets ignored over roles with more utillity. I think Destiny would lean towards the latter, as the kill times in this game are super quick. We have seen players using Radiance, the Firesinger's super to revive themselves post-death to enable them to fight on with a buff that increases their (damage?) and abillities. We don't know at this point to what degree this super will effect team mates, though the way the class was pitched it seemed like at the very lease the buff could be spread between allies, I'm not sure how I feel about a Firesinger being able to revive a ton of surrounding team mates. Hopefully the Firesinger is playable in the Destiny Beta, then perhaps us fans can get a better look at what the class entails :)