Wow, there have never been this many responses to a thread I started before... yay 
I do appreciate that lots of places have rubbish public transport- I live in London now (under protest) but I'm from the back of beyond originally, so I know it's not always possible to leave the car at home. And I'm not saying that everyone should *have* to get to races without a car- just that for those of us who have to / choose to, it would be nice if it was easier!
Personally I've travelled to some quite far-flung places without a car, an example being the South Devon marathon in Feb. This involved 3 trains, a 3 mile walk, then camping overnight and reversing the process after the race. But I really wanted to do it, and despite asking around was unable to find a lift down there.
Good points re. volunteers staffing races, and the difficulty of organising yet another thing. I think especially for larger races, it would be reasonably easy to check how many runners are in each car, because there are usually marshalls at the entrance to the carpark to direct people. And certainly somewhere to keep bags is not hard, as a lot of races prove.
I guess it will be interesting to see, if I can ever afford a car, whether I still care about this!
Hope so...