Mostly the spots arent the problem. the surface has to be fairly even, flat. If that has worn, or corroded, then you have to replace.
But mostly the internal dampening system fails over time. That doesnt meant the flywheel cant have any play in it. BMW/sachs doesnt give out any data on this, but other manufacturers do. For instance LUK (they invented the DMF) says that for the m54b30 flywheel a rotational play of 11deg (4 teeth on the starter gear) is normal and axial play can be 2,9mm. That is the data they give on product info.
There are a lot of stories on DMF's on the internet regarding that if there's play, the DMF is scrap, but that's not true. Some DMF's are allowed more play than others, Luk gives different numbers for a lot of DMF's.
Obviously if your DMF is starting to make noise or sensing vibrations, you have to replace it
