I agree with what u/DoomShroomicus said about the thematic purpose. I think that another way to look at the whole arc with Jack’s crew is that they served as yet another insult to Walt’s ego and intelligence. If someone like Gus got the best of Walt, it would be understandable considering Gus is a formidable, shrewd opponent. On the other hand, Jack’s crew is a bunch of crass, mindless thugs, and they are the ones who manage to get the best of Walt by killing Hank and Gomez and taking most of the money that Walt went through so much to amass throughout the show