I used to think it was a stupid thing to do. Then one day last year I was a passenger on a single-engine flight out of Elstree and I discovered after t/o that we were routing 2400' down the Lea Valley, over LCY then direct Lydd. Being a bit apprehensive about this I kept a good eye out for potential landing areas in case the donkey did go bang. At no point were we out of gliding distance from at least two or three good forced landing options.
Now I usually route to the east, but if I'm taking a passenger who'll appreciate the view then I'll happily ask to transit the zone.