This weekend's Rock 'n' Roll Arizona Marathon was delayed half an hour, evidently to accommodate a crush of runners caught in traffic. It's just the latest example; you run enough races and you're bound to endure a similar episode.
I don't care if it's your 1st marathon or your 100th, you don't plan to roll in at 7:15 for a 7:30 race and then realize HOLY SHIT THERE ARE TWENTY THOUSAND OTHER PEOPLE HERE!!! The vast majority of people are able to plan their day around a pretty significant event. They get up, get dressed, go through their race morning routines and head for the start with plenty of time to spare, making sure not to introduce any unnecessary stress into their preparations.
For most people, this prep work includes taking a warm-up jog, timing fluid/gel intake and hitting the port-a-potty one last time. Like most, I have a certain way I do these things that I like to think gives me the greatest chance for success. And now you wanna foul up my whole process because a bunch of lollygaggers can't manage their time?
It's just a guess, but I suspect most folks who are really interested in their times make sure to be where they need to be, when they need to be there. So starting a race on time and telling the late-comers, "tough break, go ahead and start whenever you're ready," isn't hurting anyone. This is why we have timing chips, right?