Cowl has been playing with necromancy and outsiders. A lot. I am sure he is taking all sorts of precautions, but I suspect there is a limit to how much he can shield himself from the consequences of that. I suspect Cowl's magical energies are starting to show as tainted.
Point of order, Cowl seems not to have any direct contact with Outsiders. In his fight with Dresden Harry notes that his magic lacks the usual
nauseating, greasy, somehow empty feel that I'd come to associate with the worst black magic.
Butcher uses this kind of descriptives almost exclusively for Outsider influence/magic. Stands to reason that Cowl doesn't have direct contact with Outsiders, or at worst is so skilled with his magic that he can avoid being tainted by his contact with them.
But back on the topic, I agree with most reasons stated here. Freedom to act without any scrutiny being a big one, and as most people said being the Councils expert on Vampire he would have been in high demand during the War.
His Vampire expertise is one too but in a different way. Every time we see a Cowl sponsored plot going on, vampires are somewhat connected to it.
Dead Beat - the offensive of the Red Court was scheduled with the Dark Hallow.
White Night - Cowl was the one to steer the Skavis into betraying his Court.
The whole plot showed a deep understanding of White Court Vampires psychology, which as Ramirez mentioned is NOT common knowledge amongst Wizards or Warden. It is actually so suspect to understand a White Courts psyche that it was enough for Ramirez to suspect Dresden of backdoor dealing with the Court when he showed it. So this definitely implies that Cowl is a Wizard with above standard knowledge of at least the White Court - say like Simmon the Council expert on Vampires.
His style of magic, but this one can honestly go both ways for Simon or Justine so I guess it doesn't really count. Harry uses an x shape of his wrists as a defence posture, so does Cowl. If Simon taught Justin and Justine taught Dresden what he knows about battle magic. It makes sense for both of them to have similar gesture. And no, crossed wrists are not a standard position because we see Peabody in Turn Coat using the Doctor Strange gesture as a defensive Sigil.