I try my best to always look at the light that is present and go with it before I use a flash.
When Nikon introduced the Nikon D3 the ISO on that camera went to 6400 with no trouble. Then Nikon bumped it up again with the Nikon D3S to ISO 12,800.
My newest Nikon D4 ISO goes to 12,800 comfortably and if you really need it you can bump it all the way to Hi-4 (ISO 204,800). The Nikon D4 has other improvements as well with focusing and especially in video.
In the photo of singer Marc Brousard singing at the People's Fest at Atlantic Station the lighting for the stage was very minimal as compared to other music events. For most other major music events at Chastain Park Amphitheater or Verizon Wireless Amphitheater you could easily shoot people on the stage with ISO 400 or 800.
This is important because for the bigger venues you can actually own a much cheaper camera than you need to capture the event when the light is low.
This is true in sports as well. When you are photographing an NFL game at night the light on the field is much better than when you are shooting in a small town at their high school Friday night football game.
Do you need to go and buy a Nikon D4 for $6,000 without a lens to take photos? If you want the very best ISO 12,800, but Nikon even makes point and shoot cameras that will go to ISO 12,800.
If you want to shoot events like the People's Fest at Atlantic Station as I did this past Saturday night then you need a high ISO camera. Minimum of ISO 6400 in my opinion will just barely work before you need to add a flash.
For the photo of the couple I am hand holding a 70-200mm lens at 1/25 and vibration reduction technology is helping, but if they move they would not be in focus. The light from the stage and surrounding shops is helping light them, but barely. ISO 6400 would have me shooting at 1/10 of second. I doubt seriously that I would have gotten this shot without the ISO 12,800.
With the recent dismissal of the entire photo department at the Chicago Sun-Times we can see the times are changing. But there is no way an iPhone given to a reporter will capture the photos I was getting. Also, just to own the Nikon D4 will not get the same results.
You have to know how the camera works and how to make it work for you to get the best photos.
To remain competitive you need to have the best gear to capture the type of photos of the subjects you want to get paid to shoot. If you don't have the best possible the guy that is able to show a better image because of their gear will get the job.
Remember, it is the photographer with the best images of the subjects that will get the job and not the most expensive gear.
All this is to say, your images decide if you get hired not the gear, but your gear can hold you back from getting good images. If you have great gear and not good images then you still may not get the job.