I actually did the hike from T3 to MSC where the AF lounge and gates are a few weeks ago because I wanted to check out the AF lounge before my VS flight. Even with the moving walkways, I found it to be quite a trek, and my SO was quite irritated ;-) I wouldn’t do it again.
I would think a very small minority of AF flyers would choose to walk all the way over to T3 just for a lounge… and plus, we already know that there is going to be better champagne in the AF lounge.
Well, some of us might be connecting from DL, so we might want to suffer in the D1 lounge before embarking on the AF Preflight Exercise Program(me).