Ooh nice you got a DC50!
Do you feel it vibrating the carpet with those sort of beaters?
I heard a lot about the carbon fibre wearing out, I think it's caused by people not clearing the brushbars regularly, so the tightly wrapped hair/threads/strings bend the bristles and thus wearing them out. When the brushbar is cleared, the bristles are worn and in use they have to cope with cleaning the carpet at the high RPM of the brush motor. This makes them even more worn out IMHO. Yours looks like it has had a lot of material wrapped around the brushbar even the red nylon bristles look well used
I've not had a problem with the DC35 we used to have, my DC41 Mk2 has felt around thesoleplate which is still like new since being used as our main vacuum since February if that helps.