For me the reason why I can ditch most of my spirit in a 10 man tank healing situation and rely mainly on cooldowns for mana is the group setup and trust in the healer/s beside you. Not that I slack much but I don't need to go crazy on my mana pool, my self and the Druid cross heal a lot, so over all the raid stays healthy and we pick up slack for each other when times are hard.
Because of this both of us can relax and use less mana intensive methods to keep everyone alive, it results in both of us having more mana and having a lesser requirement on spirit in general for most fights.
But with that said we tried baleroc heroic tonight single tank 2 healed and on one of our best tries to 35% both myself and the druid where spent on mana but the shard people failed anyways. If it was only me who had no mana left then I would agree it was due to my lack of spirit but the fact that both of us where on the same amount told me we where doing something wrong as a group.
A few tries later we got him to 35% and failed to the shard debuff again, but this time I solo tank healed till we wiped, I had 90 stacks of the heal buff the druid had 129 stacks and both of us where around 50% or more mana with cooldowns left over.
My point of this is when things go like clockwork healing is easier and in my raid group we tend to have things go right more often then not. So the amount of spirit I need is completely based on the group and at the moment I need very little.