He's 46, a Hall of Famer, and has held titles in two different weight classes (Light-Heavyweight and Heavyweight). What more could he possibly have to prove? I mean, if he still thinks he has gas in the tank, fuck it! I say go for it.
Wouldn't mind seein' him as a commentator. At least then, he'd still be involved in MMA in some capacity.
I figure once you reach Hall of Fame status, regardless of the sport, you've pretty much cemented your legacy.