A front mounted wide converter will most likely reduce image quality by introducing new elements into the optical path, increasing the change of flare, as well as increasing the amount of refraction occurring which will result reduced corner sharpness and more distortion/aberrations (unless the adaptor is very well corrected).
A front mounted wide converter wont reduce the lens f-stop (a rear mounted wide converter would actually gain you stops, but reduce the size of the image circle).
A rear mounted teleconverter will reduce the maximum f-stop by the magnification factor (a 2x TC would turn an f/2.8 lens into f/5.6. A front mounted teleconverter wouldn't, but would be larger. Again using a teleconverter will introduce new elements, aberrations and reduce sharpness as you are enlarging the image. Rear teleconverters used with telephoto lenses can still yield very good images as the light reaching them has very little spread.
As to why the converter is not included in the design, that is so the reduction in image quality isn't forced on people who don't need the increase/decrease in field of view. Canon have announced a lens with a built in TC that slides out of the way, which is a potential solution to this problem, though it comes at a higher cost.