Eli Gold is the play-by-play guy.
Phil Savage is the color guy / analyst.
To mix a lot of metaphors, Tom Roberts' role is the utility infielder. He's the mortar between bricks. The straw that stirs the drink. As in the old BASF commercials: he doesn't make a lot of things....he makes them better.
I think one day, when Eli hangs it up, Chris Stewart would be the one to take over his duties. But he doesn't have the game experience to be the color guy in today's broadcast world. Doug Layton could do it back in the day, but the audience now simply demands too much analytical capability.
Meanwhile, Stewart's a great choice to sit in Roberts' role.
I could see GMac in the color chair at some point, but I think he'd have to choose between being on the SEC Network and being the color guy for an SEC school's radio broadcast. Something tells me the SECN pays a lot more.