Other than with an Earth Wizard, I personnally don't see which mage can benefit from turn to stone. The only other mages proficient with Earth are the Warlords and this spell will cost 7 SPB! Not worth it if you ask me!
So, Earth Wizard it is: Moreover, if I am an Earth Wizard and I want to incapacitate an enemy creature, I would still go for Banish, because it can't be dispelled and thus gives you 3 solid turns without interruption.
Yes, Banish is a bigger investment at the beginning, but Turn to Stone will cost the same after three turns, if it stays three turns. By then, I'm pretty sure it's either dispelled or destroyed willingly by its owner who decided not to pay its upkeep cost. And if, by any chance, it's still there after three turns, than its controller is just not playing well because he just played to the equivalent of two Mana Siphon on himself!
Not to say that it's a bad spell. It will still put pressure on the opponent, and it will definitely neutralize some big threats temporarily, but other spells just do the same for less.