It depends on the kind of upgrade you look for. Like others, I have been frequently upgraded on AF European flights, as FB gold. Even if I pay the lowest fare, I stand a good chance to be upgraded to Tempo challenge or business ; I would say 30 to 50% chance based on past experience on 30 or so European flights (I always ask nicely when I get to the lounge). It is not a very big deal, but if you fly every other week LHR-CDG-LHR (meaning some 40 segments) that would be a significant advantage.
Also, as FB gold, you are offered the front row of the Y cabin, when you OLCI, even on the cheapest fare. It is fun to disembark faster than the back of the plane.