DS and Poser use entirely different weight mapping techniques, and rigging and morphs are the most time consuming part of content creation. Not quite double the work to make it work in both, but commercial products are expected to meet a higher standard than you might expect if it is for your own use. 

Using genesis clothing on Poser figures is another challenge, as the clothing would have to be reshaped to fit Poser figures and THEN rerigged and regrouped.

After all that is considered, content creators have to weigh whether nearly double the work would result in nearly double the sales.