I'm really puzzled by all the Arthur love, because that was the shallowest character of all. Also, he cocked up his job twice in the movie. JGL is hot and all, but wow.

Perhaps Mal and Cobb were either both extractors or both architects; they wanted to work together because they loved each other, so one (or both) of them needed another skillset. (Also, there's no reason Mal couldn't have been a point man; Arthur can do architecture too, we see him teach Ariadne. They could all have doubled up when necessary.)