As I said, Thaurissan is indeed a boon for combo decks that hold a large hand in anticipation for that big end game burst and yep, Freeze Mage sure is one of them, my point was that Thaurissan is not that useful outside of the combo deck.
The problem with tempo decks going into the late game isn't mana, but rather card draw. Saying that it speeds up your game to miracle rogue like levels is false since what make miracle rogue work is the auctioneer draw engine which makes such hand vomit possible. A typical mech mage would have overwhelmed the opponent on the board by turns 3-4, and Thaurissan isn't going to have effect in this deck because by turn 6 most of the minions would have been played already, and the mage would rather Arcane Intellect than use Thaurissan to reduce the cost of her non-existent hand. Turn 7 is way too late for a mech mage to start stabilising on board.
As a midrange deck you rely on dropping stuff on the board to control it in the early game. If you're a midrange deck and you have a large hand when you drop Thaurissan, then you're doing it wrong because you have probably lost the board already. Thaurissan is not a good play if you have lost the board since even if you manage to hand vomit on turn 7, the opponent is either already pushing for lethal, or can use spells plus his existing minions to remove whatever you just vomited.
Furthermore as I said, turn 7 is enough mana to make big plays in a tempo deck, Thaurissan isn't saving you any mana but just taking up a deck slot.
And no adjusting the curving is not valid since that means putting in more late-game stuff and ignoring the early game and by then you're not playing a midrange/tempo deck any more.
TL;DR: Why Thaurissan isn't useful in midrange/tempo decks -
Drop Thaurissan and have a large hand: Lots of value but oh fuck I don't have a board
Drop Thaurissan and have a small hand: I have a board and I can play what I wanted to on turn 7 anyway so Thaurissan was fucking useless