I've seen a number of people post here (not this thread specifically) that "disc is competitive" because they managed to keep up with other healers. Then you look at the numbers and find out its a disc priest operating in the 85th percentile comparing himself to a shaman in the 50th. Disc "snipes" other healers, which means it always looks better on the meters than it is until there's too much to heal. Aside from Malkorok, I very rarely find myself looking for my single-target direct healing buttons and PoH is nearly useless outside of SS. In any situation where I might have found myself using those abilities in previous tiers, I just shrug, hit the boss with pennance or look for someone worth putting a shield on and let the other healers handle it.
I think our utility is enough to make us valuable in 10s, but I wouldn't pick disc over another healer for 25s on the merits of spec. And when including them, you should understand and plan for the fact that they trade raw output for utility. Atonement can't really be free of cost or no one would run without disc and I understand that, but I think they've made a minor mistake in how that's balanced. If I were looking at a particularly hard healing fight and not considering the damage component of atonement, I definitely wouldn't go for disc. And if anyone doesn't think atonement damage is the biggest reason to bring disc, you can validate this by the number of disc priests using Yu-Lon's cloak.