There's a similar thread going on on Touring & Tenting, and we now have some input from one of the members who works (or worked, I'm not sure) for the fire service.
Aparently muppets blocking the hard shoulder is a common enough problem as it stands - when the motorway grinds to a hault, some will ignore the law and use it as an extra lane to try to reach their destination.
His post seems to indicate that people will make room for the emergency services one way or another (each lane is, after all, wider than a car) though having a whole queue of traffic in the inside lane would make matters worse.
Linky here for anyone who wants to read (I'm FishyDave in case anyone hadn't guessed.)
Driving on the hard shoulder at busy times - Touring and Tenting
Looking at the results on the M42, it looks like this has improved overall safety on the road. I suppose you have to offset the decrease in risk against the increase in difficulty that the emergency services might have in accessing an accident.
It's kind of ironic, but my only relatively serious rear-end shunt happened on the M42. It was caused by stop-start traffic, or more to the point, the driver two cars behind failing to realise that the cars in front had stopped. Much as I try, I can't remember whether it's on the stretch that now lets you use the hard shoulder at peak times or not.