You won't have to dual wield as Frost, though it does look like it will do better threat if you talent for it.
The pidgeonhole is self-imposed. Do you need to min/max your threat? If so, you will get a buff with dual wielding. Notice that they aren't taking away anything that 2h Frost DKs have right now, so your threat/method will remain the same, and if you're fine now there's no reason to expect that to change.
This is a popular attitude, if it *can* be "better" (read: different) another way, I MUST do it that way. Leave that to the out-moded, hardcore, min/maxing community, play it the way you enjoy it most.