I think Muller is well liked by the players, so he offers that aspect to the coaching staff - a guy who can provide motivation when needed, allowing Martin to focus on being the boss, and less of a "friend". Of course, I'm not familiar with Martin's exact coaching style in terms of how he builds relationships with the players, but I figure he'll be pretty strict from the get-go.
Jarvis always seemed to be Gainey's guy - but I never really heard much about what he brought to the table.
Here's hoping the Habs can find a former NHL defenseman as the other assistant - or at least someone who can work with the defensemen a bit better.