First, I would like to settle a few things:
Trujillo was very well known before Metallica. In the 80's Suicidal Tendencies was huge and his work in Infectious Grooves was also well known. We played with Ozzy for a number of years before he and Newstead did a do-si-do, but is mostly remembered in this period for screwing Bob Daisley at the behest of the Osbournes.
Primus is a band with a hardcore following, and Les Claypool is beloved by a lot of players, but I'd put Rob higher in the food chain than Les.
Now to whether or not Metallica is using Rob to the fullest of his abilities. The quick answer is no. Rob is a very capable bass player as is demonstrated by his Infectious Grooves work, however anything beyond basic finger or pick playing is outside Metallica's wheelhouse. They've made made a career of being a certain kind of band, and slap or tapping techniques just don't fit into that equation. James, Kirk and Lars have done more to make more very talented bass players sound like @$$ than nearly anyone else. Cliff's brilliant playing is barely audible on their first three albums, Jason's track is literally non-existent on Justice, and anything after that is of little consequence. Rob's sound on Death Magnetic is a muddy mess, which is appropriate since the entire album is a nightmare spawn of the loudness wars. He could play better if they let him and he could sound better if they let him. I guess as he sees it he is playing bass for Metallica and getting paid a ton for it; so just smile and wave Rob, smile and wave.