I look at it this way: If a fellow faster bicyclist wanted to pass you, would you block them? I doubt you would, nor would you probably attend an event where you organized the D group to start before the others and not let them pass.
Could many drivers be more courteous? Certainly. Should infrastructure be changed to be more bike-friendly? Absolutely. Does pissing off drivers by blocking roads do more harm than good to these causes? I certainly believe it does.