I agree partially, I find holy's mastery a bit underwhelming. When they anounced the concept of mastery I expected something silimilar to a healing proc trinket: every heal has a % chance to proc an intelligent(or not) heal to a random nearby ally in proportion to the amount healed, or something like that. But with the time I've gotten used to it and came to like it.
I'm not math theorycrafter, but I think that our mastery is actually better than it seems but it's not very noticeable since it's a hot that heals for (usually) low quantities. That said, I still prefer haste over mastery since it helps me to react faster to unexpected damage.