Well, I kind of have this fantasy storyline running through my mind which involves the location of Jack and Ennis being discovered. They have to go on the run, relying on both Ennis's FBI training and Jack street awareness to keep them alive.
Seeing as Ennis has been protecting Jack so far, Jack will have an opportunity to save Ennis's life (when it would be easier for Jack to desert him).
I've been thinking about this KAren...
Given the amount of effort Jack puts in to keeping himself fit in order to outrun Madrigal and whoever else, it might be that he and Ennis get into a situation where Ennis's lard-ass ways put them at risk. Like you say, Jack could easily desert Ennis, but he doesn't.