Ironically (I mostly play as rogues), Cole's skillset was exactly what I would have expected out of a Rogue/Mage class cross. There was a time on the older forum before the game was released, that in DA2, it was conjectured that regardless of your class,Hawke would ultimately be a mage. A roguish mage might be able to become effectively invisible or change appearance, and depending on good/evil alignment be able to sway the thoughts of others.
I'll freely admit, Cole has the potential to have a very useful skillset, in combat and outside of it. I don't see it happening early, but if the Inquisition is made aware of Coles real history with no sugar coating (ie it is made abundantly clear that it did habitually killed innocent men, women, and children in cold blood for what amounts to the thrill of it), I'd probably turn Cole away. I can imagine that shoe not dropping until well after Cole has established itself as a valuable part of the Inquisition. Deciding Coles fate at that point, hopefully, will be an option.