I think Todd was used to show Jesse and Walt, and the audience, that now they're doing serious crime. Todd, IMHO, is a seasoned criminal and follows orders very carefully.
>now they're doing serious crime
They've been doing pretty serious crime since the first season, especially the fourth. But you're probably right, the thing that makes it so good is that Walt and Jesse are so charismatic. The audience almost forgets they're criminals with quite a few murders under their belt because they both have their reasons.
Also the last episode, god damn. I nearly jumped out of my chair. I'm so excited for the next episode