Frost Strike doesn't apply disease, because it is the runic power dump.
Same deal as Rune Strike or Death Coil.

It's a toss up really.
Do you go Necrotic Strike (1 Unholy) or Obliterate (1 Frost & 1 Unholy).

The two different modes is a key element.
Either an IS Boxer toggle, with the same spam key.
Or two different hotkeys, calling slightly different spells.
That way you can switch from AoE_Pressure/Slow to CC/Focus_Fire and back.