You want the wheels on a bike to spin as freely as possible, any added resistance is unwelcome because it makes cycling harder - and you would need to add resistance to generate electricity. Personally I even hate the dynamos that power the lights, because their effect can definitely be felt when you ride.
Modern electronics don't really need that much, and/or the dynamos have improved. I had a bike with 90s-style dynamo and it was indeed a slight pain. Today I ride Bixi/Alta bikeshare bicycles - they have something to power the lights, but any impedance it creates is too small to notice.
Hub dynamos are very efficient. I do not feel a difference riding with or without one. For street speeds of around 20km/h in an urban environment they create mechanical drag of around 6 watts [1] with the lights switched on.
