If you are an O' Connor fan I would make the stop. The drive on I16 to I75 is mostly farm land and pine trees and it takes closer to five hours.
I just don't understand the routing. It's not a natural detour with Americus as the starting point. The drive from Americus to Savannah is just a little over 3 hours and there's a lot more to see in Savannah then Milledgeville.
If you DO want to detour north and add to the drive then there may be other places to see instead of or in addition to Milledgeville. Eatonton has been mentioned. Madison is one of the 10 prettiest towns in the US. Juliette GA is the tiny town where Fried Green Tomatoes was filmed. If you are a fan of the Vampire Diaries, Covington is the location for the show. Social Circle is an historic little town with a great Southern buffet, The Blue Willow Inn. Lake Oconee may be of interest. Macon has some things to see that you may be interested in.
What are your goals? What kind of things would you like to see? How long to you want to add to this drive. An hour? An afternoon? An overnight?