Hearthstone 20: What is your Quest?

[Zevox]

I don't think that's disagreeing, as such. Either way you're playing with the new cards without buying the new cards. The advantage of the random ones is that they showcase the new cards which might not make it into competitive decks, while the advantage of the pre-constructed decks shows the fun interactions.

But with random decks you're not necessarily showing the new cards, because the cards are random. You can just as easily get little to no cards from the new set, by the very nature of the beast - especially since, if memory serves, Tavern Brawls are Wild, so the new cards are far outnumbered by the old. I think that would make it poor for showcasing anything.