When I upgrade my Cloak I will be placing Spell Pen on it. When I feel my resil/stamina is at a sufficient point, I'll place some gems adding more Spell Pen. It is quite a valuable assest to our arsenal but nothing that takes priority.
The 35 that a cloak offers pretty much negates a Mark of the Wild, the problem is Arcane Mages with a base resist of 140. Or a hunter that throws up Aspect of the Wild. It would be nice to have 100+ spell pen, but other stats at this point in time are too valuable.