The biggest thing for me, is the 0.5 second reduction in the Global Cooldown (GCD).

The majority of our moves (almost everything except Hungering Cold) is instant cast.
Our attacks (as Frost) are either Howling Blast (Frost Rune), Necrotic Strike (Unholy Rune), Either on a Death Rune, and Frost Strike (or Death Coil) on Runic Power.
We basically never have a point where no runes are available and we're out of Runic Power.
So there's always something we can use to attack with.

In any presence other than Unholy...
We have an instant move, and then 1.5 seconds of nothing.
Over 15 seconds, our attacks might be:
00.00 Howling Blast
01.50 Howling Blast
03.00 Howling Blast
04.50 Howling Blast
06.00 Frost Strike
07.50 Necrotic Strike
09.00 Frost Strike
10.50 Howling Blast
12.00 Frost Strike
13.50 Frost Strike
15.00 Necrotic Strike

Meanwhile, in Unholy Presence and over the same 15 seconds, we have:
00.00 Howling Blast
01.00 Howling Blast
02.00 Howling Blast
03.00 Howling Blast
04.00 Frost Strike
05.00 Necrotic Strike
06.00 Frost Strike
07.00 Howling Blast
08.00 Frost Strike
09.00 Frost Strike
10.00 Necrotic Strike
11.00 Frost Strike
12.00 Howling Blast
13.00 Howling Blast
14.00 Frost Strike
15.00 Howling Blast

Having your five Howling Blasts, and four Frost Strikes each hit for 10% additional damage (from Frost Presence)...
Might add up to as much as an extra attack.
However, three additional Howling Blasts (or Necrotic Strikes) and two additional Frost Strikes adds up to a substantially greater amount of damage.