Dragon priest wants to wipe the board on turn 4, so aan early turn card that gives something other than a body is good. It's not netherspite historian good though, the thing that made that card great was a) there are very few BAD dragons, not enough to get three bad choices, and b) it keeps the dragon train running. It could proc off of operative, who could then proc off of the card discovered by netherspite, etc. It allowed you to play a couple less dragons and still always reliably have one in hand.
I feel like the card would shine in a Dragon Mage. Most spells are pretty good and Discover-a-spell minions were played by Mages in the past.
Just a wild thought, perhaps a Big Spell shell could adopt some Dragons? They play mostly good-stuff cheap minions to hold the board, they can easily replace most of them with new cards. Faerie Dragon, Nightmare Amalgam, Sindragosa and Alexstrasza can find their place in the deck, so it would need another Dragon synergy card and another good Dragon to be consistent, preferably ones that cost 3 and 4 respectively.
it probably goes on Paladin just because it has draogn sinergy cards and rather strong spells that you dont want to waste a slot on, like when priest used to play Light of the Naaru, like you wouldn't play Lightwarden but you were happy when you got it for free.
Big priest was never a problem until Lesser Diamond Spellstonegot printed, and this spellstone breaks way too many decks asides from big priest, basically compare it to gallery and you can see that spellstone should cost more than gallery, barnes isn't a problem in any other deck, spellstone tho, spellstone is a problem in many different priest decks both in standard and wild.
2/2 psuedo-draw for 2 has rarely been bad, and it won't start now. Discovering is much better than an entirely random spell; at least this way you get the least bad (and hopefully actually good) of 3 options.