Cinematographers get very tied up with technical stuff, and worse yet, gear. There are so many nuances in cinematography, and filmmaking in general, from the creative to technical, For a DP, creative tasks usually consist of analyzing the script, and doing everything in their power with camera composition, lens choices, and lighting design to compliment this story that has already been provided for them. Generally speaking, it would be unnatural for a DP to be a really good screenwriter.
In theory, with the availability of increasingly capable and decreasingly expensive consumer filmmaking equipment (and I'll go out on a limb here and say most contemporary cinematographers can easily get their hands on a good camera setup and able mic these days) we can, and should always be practicing our craft by making our own films. As cinematographers, we are in a competitive field, so unless you're so booked up (in which case you you are not even reading this blog post) if you do not practice, you will be left behind by your competition. A young soccer player will lag behind the rest if he doesn't go out and practice every day, why do we think this won't happen to us as DP's? And I know what your thinking: we aren't good screenplay writers ... so are we just supposed to go out and shoot boring camera tests all day?
I'm calling all DP's to always be working on a documentary. Find a subject you love and make a doc on whatever that is from a creative viewpoint. Make a doc on a hobby. Make a doc on someone strange in your life. Make a doc on some small interesting detail you find interesting in your world that without your documentary, no one would ever have paid attention to. You can literally choose anything - find something to focus on and go out and shoot. Remember the point of this is to sharpen your blade as a DP. The only rules are to get creative with every aspect of your film, and when it's done, to publish it. Oh, and it better look damn good too.