None of the parking in the East lot is really close to the main entrance. The West lot is closer but was not open on either of my visits. If I am going to be walking 5-10 miles a day, I'm not really concerned with a few extra steps. But if you have it free, I can't see the harm. If you have to buy it, I'd say it's not worth it at all. YMMV if you have small children or elderly with you.
As far as leaving and coming back, I'm not sure. I have a diamond membership and did not use preferred parking at any of the six SF parks I visited last year. Traffic in Atlanta (even on Saturday) is awful and I would subject myself to that as little as possible.
Traffic in Atlanta is horrible no matter what day of the week it is.