I always look to the "not in my job description" work as an opportunity to grow, either by acquiring new skills for use within the organization or to use on my resume for my next organization.
With "lots of down time" I would question how many of the agreed upon hours were actually performed each week and if what you call off the clock time is actually off the clock. In organizations I've run we don't do 9-5, in fact I don't really care when most people work as long as they actually __work__ and get their projects accomplished. The stars in my teams are the people who take it upon themselves to go out and find ways to improve the product, present them in a clear well stated proposal and then can work to implement them. The people who wait for work to be handed down from above aren't headed anywhere.
Think of the IT system as your product, you may work mainly in 1 area but you're clearly smart enough to think of ways to improve others. Its all about attitude, and this attitude that you're entitled to extra cash for working in other areas even though you already have "lots of down time" would cause me to run not walk you to the door.
Now with the attitude of pitching in and working to make the whole thing better, I would look at you as someone who "gets it" and probably look to help you move up in your career track provided you had done other impressive work. If you're looking to be an IT director some day this is the kind of work you'll be needing to do day in day out not just what you're asked to.