For the same reason that my graphic artist friend Orijit Sen (Mapping Mapusa Market) insists on drawing sketches instead of taking photographs. To use his words - sketching allows you to dwell on the subject and hones your observation skills - in turn the image (your sketch) ends up more like you see it - the unwanted clutter is kept out and the stuff that matters gets detailed nicely. Similarly, a well taken photo will bring out the subject matter better and in a more focused/targeted way - the subject is still and not moving - the composition, and the pan and zoom can emphasise the narrative further. Where necessary, appropriate effects (e.g. sepia toning) can be also be applied.
An amateur video is an easy way out of coming to terms with learning the visual language. One may think the it is more effective, but in reality, it may end up being just the opposite. Composition for a moving image is quite different from composing a still photo, and requires different skills not just in composing but also in editing. I suppose your students may be talking about using brief unedited footage, and I am not saying that they won't be able to manage that, just that when using still photos seems more appropriate, why use video?
I have to add that the part in your email about "fancy effects" is already making me cringe :)
Salil.