See http://www.idrawboxes.com/2008/yes-no-questions-dropdown-radio-buttons-or-checkbox/ and http://www.isocra.com/2008/02/poll-a-yesno-question-in-a-web-form-which-controls-would-you-use/ for further information.
One other comment here, something I find frequently confusing is the use of a single checkbox, usually empty, with a Default statement. Consider the example screenshot.
Usually, a checked checkbox I think of as Yes, and an unchecked checkbox as No. I believe that is the intent in these cases in Moodle. However, given the many settings in Moodle, suppose I come back to this page later, and can't remember whether I checked or unchecked the box in the screenshot. If I check it now, does that mean I want the default No, or does it mean I am checking the box to say Yes? If it is unchecked, am I safe to assume that unchecked is No, the default? To me, I think it would be clearer to use the radio buttons to have it always clear that the value is set to a specific state.
I guess I would ask the question does the default value in these cases add any necessary information? I understand if the goal is to display a recommended setting, especially in when it applies to security settings. For example, if you deviate from the recommended security, there could be consequences. But in the screenshot case, is it actually important for me to know what the default is? I would argue no. I only care that I can always tell what the value is set to, and that it is clear what will happen if I change the state of the control. In the case of the screenshot I included, if there were no default text, then I think it would be clear that the checkbox is a yes/no, or an on/off.