Max damage by squares in total AoE is really a poor metric to evaluate AoEs. Encounters are encounters. You may see 10 - 12 mooks, at most, in virtually all of the cases where an AoE will be brought to bear. Amongst those 10-12 mooks you may, if you're lucky you'll be able to deploy it against 3/4s of them (unless they are oddly in fireball formation). Fireballs AoE spread is just about the upper limit of necessity and further, it is about at the upper level of tactical wieldiness given the demands of positioning and avoiding friendly fire. Further, Cone of Cold is a Blast/Cone so its range is "Melee". As such, it requires the Wizard to be in a compromising position (in range of enemies melee attacks). Whats more, in most cases, if the Wizard is in melee, amidst the enemies he is "coning/blasting" will also be his allies. If the spell was (as 4e) Target; All Enemies in Blast then it would be much more wieldy. However, as is, in almost all cases, Cone of Cold's extra AoE spread will be useless and in some cases it will be worse than useless - tactically unwieldy due to melee requirements and friendly fire implications.
The only time my player's have typically used Cone Spells, in 3e and prior, is when playing Fighter/Mages or when they had immediate get out of jail free contingencies available. Wizards in 4e used Blasts-aplenty.