I've just had a talk to "Ben" my 11year old red cocker and he says..."Please leave my little bits alone, thnk you very much" 
I've just had a chat to my Buddy and warned him that he must either keep that hair clean himself or I will come at him with the scissors and give him a teensy trim!

I do usually trim it, for all the reasons listed previously. But I do leave some as I'm sure it's there for a reason.