I think the most important thing to be taken away from all of this is that Disc if anything is one spec where balancing stats is as useful as it will ever be. The thing with Haste, Crit, and Mastery, is that they all synergize very well with eachother. Which is, in essence, one of the reasons it's hard to define a "right or wrong" scenario with Disc. Secondary stats are all beneficial to us - Prayer of Healing double-dipping in Crit is not by any means a small thing to consider (After all, I think most of us use Inner Focus for the crit bonus, not so much the mana saving, these days. Long gone are the days of Inner Focus -> Crit regen 5secondrule > woo mana return, ala TBC).
I would also very much agree with the Haste argument at the end of your post - this is also much the reason Haste is not my most focused stat - Mastery is. My train of thought is pretty much this; Haste is good in that it provides more spells cast during the course of an encounter - mana costs aside. Crit is also good in that it provides more Divine Aegis, whether that be from pure crit sources, or Prayer of Healing double-dip. However, Mastery is one stat that benefits both of these scenarios. It's the jack of all trades, master of none. And do take that last part in consideration; master of none. If you're not spamming PoH, Mastery is not a great stat, full stop (compared to Haste, anyway). Whether this train or thought is optimal or not, it has landed me a #5 ranking for Disc healing on WoL, and a #22 over -all classes and specs-, for Zon'ozz Heroic. Does this mean anything substantial? No, not really, there are too many variables to consider. But on the other hand, it should mean that my approach to healing is not awful, or bad, or useless. It is, at least, useful, I hope.
But really, the take-away is, as I already said, focus on what works for you. In the end, it's how you play your character, which determines whether you kill the boss or not - your focus on Haste or Mastery (or Crit) is not going to be the deciding factor. Once you get to the level of play at which your stat allocation and fine-tuning matters, you'll already know what matters most to you. Follow that, and not what is said on this forum (Including what I'm saying, it's worth no more than what anyone else said in this thread).
Just enjoy Disc in DS. It's a good spec, I can attest to that - whether it's the best spec or not, and whether I can answer you questions or not, is not the most important thing.
Drunken forum posting ftw \o/ happy 2012 everyone.