I agree you could wonder why a developer would use !important in modules CSS in external files or inline.
My initial post was about the actual problem, not the concept.
BTW, there is no !important in DNNs "default" default.css afaik.
If the module load a CSS file and it's loaded before skin.css (it should) then you can overwrite any value that's defined with !important by using !important in skin.css.
If that doesn't work either, please post a link to an example...