It doesn’t matter. If you want to stack cold onto the enemy so it takes more damage, it’s a debuff-centric style that’ll always be inferior to the alternative and always have a lot more hurdles to both design and gameplay, and with all of that I didn’t even mention problems with travel time of projectiles. I don’t think theme should be above gameplay, and this is probably where this discussion should end since you think otherwise, unless you can come up with a good alternative that would address both concerns.
You are still tying the functionality to the theme, so it’s basically the same thing. Shatter is to increase the critical strike chance against enemies that are frozen, and both Fingers of Frost and Winter’s Chill treat enemies as if they were frozen. But they aren’t. It just treats them as if they are, because for balance’s sake, we can’t freeze every enemy, which is also something that trumps over the theme.