It will be changed to spell power, so that it's useful to Shadow Priests too. Regarding the charges, they're likely to stay. The spell is dirt cheap because of the charges. If we removed them, we'd have to up the mana cost. A Warrior who keeps his shout up in fights is usually regarded as better than those who don't, similar to a Priest who keeps his Inner Fire up vs those who don't.
Unlikely also that it will be made undispellable, we're not even sure if we like the Mage/Warlock armors being undispellable. The dispel system in general (junk buffs, RNG resists) is something we'd also like to address as a whole.
It WILL affect spell power, so it is useful for shadow priests.
Not with latest changes. 120 spellpower on top rank so 3 talent points for 36 spellpower at lvl 80. Also lets keep in mind you gotta sacrifice some talents in shadow to get there since you wouldnt normally go that deep in disc. Pretty worthless talent for all spec pve wise, yet gotta see what melee dmg migration you get from the talent for lvl 80 pvp.