Well, as I see it you can make Miracles available to all attuned creatures within, which is bad enough. Basically you don't need to make anything else available, Miracles will do fine. The CL is a bit low, only 17 (or I have to pay more), but hey, I'm not using them for offence. By strict RAW, there would not even be a cost attached, since somehow it never mentions either components or XP costs. OR whoever casts it has to pay the cost, which is still fine, because you can emulate so many spells.
I've gotten a first draft of the Mythal down, it's around a 119 remaining DC, 40000+ XP and about a million gold. In this campaign there is neither encounter XP nor encounter gold, and the DM has gotten stingy recently, so that's actually a very significant investment - over one level, and I can't buy anything, either. If we take down Halastar I assume we're in for another fat payoff, but we're not quite there yet. While it's nice to have infinite miracles in one place, I'm not sure it's worth the investment right now, especially since it's a stationary thing, and it won't solve the adventure for me.
(Actually, NeverGetDrunk..., I don't use the Shadow weave, YET! It's an option I'm thinking about. Shadowcrafter is pretty nice for a few bonus feats IF I want to use it, that's why I was mentioning it.)