It's a parallel to Brian NOT fucking the client in season 1 for the job. (The tyre guy). I thought that was such a great, big thing for Brian's character development in season 1.
I don't know if they were trying to make a statement about Justin's youth and inexperience by having him actually go through with it. But I don't like that he did. It doesn't reallly seem to me something that he would do. But he did, obviously, so it IS canonical.