At the same time, there’s quite a bit of evidence that suggests raw performance is not the end-all, be-all of reasons why people play certain decks. The big reason why Barnes dodged a nerf in Wild for as long as it did was because, according to the devs, Big Priest’s playrate was disproportionately high compared to its winrate. And during one look back at the Year of the Raven cards, Dean Ayala mentioned that one big surprise for them was how little play Even Paladin saw, relative to its winrate/potential.
It’s entirely possible for less than optimal decks to see play, even in Ranked, if people find them fun. Galakrond/Highlander Priest is one of such decks, because games are seldom the same from one match to another (at least for the Priest). At the same time, this means the deck isn’t as consistent as, say, Tempo Demon Hunter or Face/Dragon Hunter, and therefore performs worse than those decks on aggregate.
TLDR: playing tier 3-4 decks is a rational decision if one seeks to maximize fun rather than ladder progression.