The mass dispel nerf is going to hit us pretty hard in arenas and rbgs - our damage will still be good enough - but will our utility? I'm not sure.
Ele shamans bring crazy cleave damage now, and the best knockback - boomkins bring strong aoe damage and ursols/solar beam, and a knockback - frost mages bring great CC and the best single target burst, fire mages bring great CC and strong aoe burst, locks bring great CC and variously bring either massive single target burst or arguably the best dot cleave.
In a world where Shadow is going to go from pulling like... 100+ dispels over the course of the game down to like 20, and our ability to void shift that first enemy kill target is being removed... it's not a question of "is shadow bad?", we're not and we won't be. But, just as the recurring ret problem - "is shadow better or equal to their peers?" - does our damage - now absent all utility - justify Not bringing another destro lock, aff lock, fmage, boomkin, ele shaman, or even disc priest?
Disc is probably the replacement to shadow for RBGs - it brings all the mass dispels of Shadow pre-5.3, still contributes meaningfully to burst cycles via penance/mindbender, and counts to your healer total: not your DPS total. So if you bring a disc, you have all the utility of pre-nerf shadow, and can put a different dps in the shadow's former spot - and gain whatever utility they are bringing: net buff to your team to have your shadow go disc (or sit them for a disc priest and bring a boomy or extra dlock).
I'm obviously not thrilled about the way they're nerfing us, fortunately I have 4 fully geared alts now I can play instead - maybe shadow will make a comeback in the final MoP season. *crosses fingers*