Again, if you think you wouldn't use that mana, you obviously run with more healers than we do. Try killing Hydross with 5 healers in the raid, and then it'll be quite obvious what you'd do with all that mana. Use it.
From what I've seen in balancing raids in TBC, each healer reduces the load exponentially, to the point that when you hit 8 or 9 healers, you stop having to pot. And your job becomes a lot more boring. We just happen to have a lot of dps, and few healers, so we've learned to adapt to a situation where we *always* need mana.
Figure a 10 minute fight (i.e. Hydross, right up to enrage, just for kicks).
10 * 60 / 50 = 12 (best case) Spellsurge procs, round to 11
11 procs * 4 people = 4400 mana per person
Shadow priest, assume 900 dps consistent, with VT never dropping off, so 27000 mana restored to the group.
Add Mana Tide, for another 48% of your mana (2 drops) over the course of the fight.
Mana Spring, fully setup, is roughly 3600 more mana.
Realistically, you're never going to see that level of shadow priest return. Even with an innervate, a picture-perfect VT-up-all-the-time rotation will never happen. So round it down to 20k mana. I can use all that mana. If you can't, think about ways you could.
Now, remove the SP from the picture, and give him/her to the dps. Suddenly, 4400 mana returned is actually the *highest* of your returns, and is a significant gain to your regeneration (works out to 37 Mp5).