Gizmodo has a good technical article (Why ISO is the new Megapixel) on the importance of ISO to your HDSLR camera.

HDSLR cameras are getting me closer to my personal holy grail, to only bring actors and a camera to set. Okay, okay, I know, I know, I also need sound, props, wardrobe etc... However, the point being that a camera's dynamic range effects how much available light you can use on set, which informs how much support equipment you'll bring, how many people y0u'll need to man it and how long it'll take to set up. All of these things add up to time and money.
In low budget moviemaking you quickly learn that what is in the screenplay or director's head may not necessarily be achievable at the actual location and to create it is beyond your budget or time constraints. So you must accept it and play the environment for what it is as opposed to what was envisioned. This limitation can be a blessing because if you don't fret over making everything precious, then you can concentrate on what is necessary - good acting and enough coverage of it so the audience can emotionally connect to the characters and clearly understand their situation. You'll essentially be rewriting your movie scene by scene in the editing bay using the footage you shot on set. So the greater the camera's use of available light, the more it can reduce set up time on set. Thus, giving you more footage in the editing bay to better help shape the movie. And who among us when screening our movies for others hasn't said to themselves during a particular scene or two, "if I only had more time to get another take I would've done it differently."