Yes.
Do certain specs have more potential than others for playmaking? Sure. Are some specs “unimpactful and along for the ride”? No, and if you’re playing them like they are, that’s because you as a player are unimpactful and along for the ride.
If some specs had no impact and were just there to pad, then there’d be none at high rating.
Saying boomkin is there to pad is just patently absurd. They were an extremely good base stealer, an extremely good FC assassin with Convoke (who can then fast carry flag to your FC), and an extremely good team fighter (and no, not just for pad; Cyclone is invaluable), etc.
My brother in Christ, Ice Wall isn’t a Frost mage exclusive button; it’s a PvP talent available to all three mage specs. So your entire argument is predicated on complete ignorance.
Holy is fine in Blitz. Ray of Hope is incredibly good for keeping flag / orb carriers alive, for instance.
Healers aren’t just throughput bots. This isn’t comp stomp. They can make plays like anyone else. Their map awareness/positioning is super important.
Hi. I play at 3000-3500 MMR on multiple healers. 
What’s the next excuse?