Shouldn't an item with that much legend behind it be called Legendary? Illidan's twin blades are labeled legendary, but they're hardly the stuff of modern legend-- Illidan took them from a doom guard commander 10,000 years ago, and they've done nothing but sit in his whirling hands since then (heck, they couldn't even take down Arthas). I agree with Scott-- there's no reason those blades should be Legendary when the skull isn't. Sure, the skull might not be as powerful as it used to be, but it's still got a load of legend behind it.
And the Legendary debate isn't over yet, because there is one more big item looming on the horizon: when we head to Northrend, we'll all, Horde and Alliance alike, be looking to find the item that kicked off this Arthas trouble in the first place: Frostmourne. Will that be wieldable by players, or labeled Legendary? If Blizzard's previous experience with Legendary items is any indication, the answer may not be what you expect.