That's why I said "AS A TRIBE". Obviously Volcanic Drake and Blackwing Corrupter are good, but the only cards with Dragon-synergy are Blackwing Technician, Twilight Whelp, Corrupter, and Consort. Twilight Whelp requires a Dragon in opening hand for it to be good, so it's not. The other three are all really strong cards, but one of them is Paladin only. So why not play the deck that inherently supports Dragon cards? On Dragonkin Sorcerer, I think this card is actually very mediocre in Priest as it exacerbates the problem they have with inconsistent draws along with other issues.
Will there be other Dragon decks? Sure. But at this point I can't see anything being stronger than Paladin as far as MIDRANGE decks go. Maybe we will see Control variants running Dragons or something. Or some sort of Shaman combo with Dragonkin Sorcerer and Windfury.
You're forgetting Rend as well.
You ask why not play the deck that "inherently" supports dragon cards? Plenty of reasons. Paladin and rogue operate very different and each has different strengths and weaknesses, even when both are using dragons. As long as a deck isn't just trying to do something another deck does well, but only worse than it, the deck is justified. Potentially even great in the right meta.
If that weren't true we'd have only a single deck of each archetype because all but the "best" would just be redundant to the "best". But that is clearly not true.
And just for example, a difference between dragon rogue and dragon paladin is the amount of high tempo + strong removal rogue has, that paladin absolutely lacks.
As for priest and dragonkin sorcerer, we'll see. I think it is more than a solid card, perhaps even great, for priest. One PWS and it is a 4/8. Hit with velen's chosen and we're talking 6/10. That is pretty gross for priest imo. I don't see why you think it would be an inconsistent card. At its worse it is still a 5 hp minion for 4. If you're running about 6 spells that target minions beneficially, I think you'll have little trouble getting value out of the card most of the time. And it enables yet another 5hp minion, blackwing technician of course.
Potentially benefiting the whelp as well. People often point out - why not just run zombie chow, after all you have auchenai. Well, not every priest deck necessarily wants to run auchenai. Or maybe you want two of both or a mix of the two.