I wouldn´t go for ArP neither for frost or unholy.
As IT6 you do 2 OBs in ~20secs. FS, IT & HB don´t get anything from ArP as far as I know. So only these 2 OBs and melee profit from ArP.
As Unholy you use SS, which deals shadowdmg. The only strikes affected by ArP should be BS & PS (& melee ofc). PS doesn´t count full here when using Glyph of SS. ArP affects your strikes dmg before necrosis counts. Necrosis deals a static 20% dmg plus to your whitehits, so it´s dmg should increase slightly.
There´s a lot of ArP gear to find in Ulduar, but in my opinion other stats like haste, crit ect. are much better for frost/unholy. Blood hast the big advantage of blood gorged and most of it´s attacks are influenced by ArP.
EJ values armor penetration close to Agi. Seems like it´s just not a 'go for it-stat'.
...please consider that I haven´t followed the whole hunters blog. Maybe everything written above is just bulls**t.