As DPS I support and approve of your sentiment, but your math is completely off. A Spinel represents a 3 damage increase over a Runed Living Ruby, which is what we'd otherwise use. At 4/9 4/5, I'm at around 1680 damage raid buffed, so that 3 damage increase is about .1% increase to me.
Plugging 1680 damage into the spreadsheet makes 21.2 spell damage a 1% DPS increase for 0/21/40. Shows how well destruction warlcoks scale - as mages would probably require a few more spell damage to gain 1% DPS while dealing similar and probably even lower DPS. And I'm not even in T6 gear or I would get less relative benefits from more +healing as well.
The total raid DPS lost by focing a warlock to use a pyrestone instead of a spinel is smaller than the loss making a mage use a pyrestone instead of a spinel, so even though a warlock would gain more DPS from epic gemming he'd also lose less when giving up on the spinels. shadow priests are much harde to take into the equasion as they scale the worst, but if they won't get spinels they'd use 9 damage gems isntead and their dps also gives mana to their group which is translated into more dps/healing (or they wouldn't be brought in the firstplace).
Like most Guilds we do have the same problem with spinals, however there is an option to lower the Demand for Spinals.
Mages Wl and shadows can get one 12 Spelldmg gem from Pvp.
I do hand 500 Gold to everyone who does this, so there is quite a bit of incentive to farm one
Even had other classes Complain because they could not do the quest *G*
Lessend the waiting list by 14 Spinals.
We have a similar quest für shadowsong amethysts, some heroics dropp equally good purple gems, worth 250g on turnin.