One particular aspect that this site mentions is the need to plan how to integrate podcasts into course content and learning activities and outcome. They suggest that if podcasts are merely an optional extra resource, most students will not listen to them.
From my observations, it is generally students that fit one of two profiles that make more use of podcasts (and probably most other 'additional' resources that academics put extra time and energy into), these students tend to be those that might be struggling (often international students overcoming the challenge of learning in a non-native tongue) and committed, engaged students.
Upon reflection, with my podcasts, I probably do not do enough to make my podcasts part of a required learning activity instead of an ancillary content resource.
More thought and development in this area would hopefully lead to more students 'fitting under the main body of the bell curve' using these and other resources.