Doug,
I'm a bit surprised by your responses here because you usually have such good things to say, but I think you are wrong here. Strict adherence to rules in all cases does not benefit anyone. This is where we read that a 7th grader gets suspended for 10 days because she brought a butter knife to school to cut her pear at lunch - there needs to be some case-by-case analysis here. As runnerchick so effectively mentioned, Marla is not having the biker replace any energy that should would have expended without the biker, so it isn't the same as Casey Martin.
Believe me, I'm not for allowing everyone to compete, and there is no way you can make things equal for everyone, but Marla Runyan is a world class athlete with the only proviso that she needs help seeing where to go. Now, at some point, if she loses her sight completely, I agree that then she will have to be done competing. Then, I don't think it is fair if she is tethered to another runner, but as it is, simply getting instructions from someone on a bike such as "turn left. water on your right", that hardly seems an unfair advantage.