You started out trying to answer a question and ended up just rephrasing it. Please don't do that.
At what point in a 10 man raid is it ever worth not being Demo and going Destro/Affly instead?
On many DS-HC fights, I see Destro as performing very well, with Bane for Morchok's other half, Bane when swapping to adds on Yor'Sahj/Warlord etc.
In fact the only fight that seem's solidly in Demo's favour is Ultrax. (and possibly spine, but we won't be doing this HC for a little while in my group).
My raid setup, caster wise, has myself, a Shadowpriest (that offspecs to disc for some fights), a Mage and Healers.
How much is that extra 4% spell power worth as opposed to the potentially better damage offered by Destro on the aforementioned fights, and in a 10-man situation, when, if at all, would it be recommended to drop the Demo lock altogether in favour of better damage offered by Destro/Affliction?
Not to mention Nether Prot seeming very worth it also on many fights.