As a frost DK, there is no reason to go beyond 3 points into Unholy in a DPS role or a PvP role.
In a DPS role you would see me go 17 in Blood to get the Crit, AP and Weapon Damage bonuses to my abilities. Unholy didn't offer me much, but the ghoul was the main reason why I would go unholy, besides picking up Necrosis would see benifit with Black Ice now, or the minor strength buff from ravenous dead. Corpse Explosion is too affected by Black Ice.
Problem with the unholy tree is that most of it heavily favors unholy, while it provides almost no benifits to frost. Frost is stuck with a blood-sub-spec. Blood can dip into either tree, and unholy can dip into either tree. But frost is stuck with Blood.
Give me a reason to go unholy and still be a worthy death-knight. Because right now, unholy does not provide enough benifit as blood does by far for the frost tree.
Liked the fact that the ghoul was accessable, it would of been great for pvp/pve dps rolls. Now that it's gone there's no reason for frost to go into unholy.