You have to remember.. NBC's goal is to get you to consume as much of the Olympics as possible. If they're showing their best stuff in the afternoon, is there going to be a draw to see a highlights show in primetime? I don't see it.
Well... there is another way of looking at that. Chances are high that some people can't see it at night because they work in a restaurant or store or some other evening job, and they watch it in the afternoon. Similarly plenty of people work in the day time and can't see it in the afternoon.
If you have ONE showing, you can get say.... 10 million viewers. If you have TWO showings, maybe you will get 7 Million viewers in the afternoon, and 5 million viewers at night, effectively having 12 Million viewers watch your commercials.
Also, LIVE viewing promotes commercial viewing. So the 7 million watching it in the afternoon will see the commercials for sure, or at least the majority.
See, if the event is going to be tape-delayed anyways, and another 3 hours on top of that, I might as well make that 4 hours, watch some other DVR'd Olympics, and not watch ANY commercials on NBC whatsoever. I think with the live showing, they would actually get more commercial viewers. Delay is delay, for those with a DVR it then doesn't matter anymore your build in your own delay to prevent commercial watching.
And that might also be a very compelling reason - especially in weekends - to show important stuff live.
For myself, in 2010/2008, I watched stuff live IF it was live (and thus saw commercials). If it was delayed in any way, I just added my own DVR delay and watched NO commercials whatsoever.