I agree that Mick Mars is terribly underrated, but Motley Crue was completely a sum of its parts. Mick's sound and style was tailor made for Crue; you couldn't plug him into just any band and get a satisfying result. But what he did with Crue had moments of sheer amazement, IMO.
As I recall, he has dealt with pretty serious health challenges his entire life -- Ankylosing spondylitis, an awful form of arthritis since he was a teen.