It probably means they're preparing for the release of some pretty powerful ice attacks or creatures that can use ice attacks at some point in the future. I mean, if they only released cards to deal with frost attacks in the expansion where frost attacks are released, then anyone who wanted a way to deal with frost attacks would have to buy that specific expansion. While you could just use defenses, more armor, guards with intercept etc., I suspect that that won't always be enough. There's probably some sort of special mechanic that will involve frost attacks. I'm guessing it will be something that influences movement or actions. So frost -x would be useful for lowering the effect die roll so that a creature is less likely to be slowed, restrained, or incapacitated. It's just speculation though.
It would be cool if there was a mage that wins by "Freezing" the arena. Like racking up Frost counters on creatures and conjurations, where 1 counter gives them lumbering, 2 gives them slow, 3 incapacitates them or prevents them from activating abilities. That would be really cool, and it would explain all the Frost -x traits, since that would be a pretty powerful effect if it weren't for traits like frost -x to lower the effect die rolls.
