The SRD takes fairly good care of dust and scratches both on the neg/dia and on the lens that was on the camera (or inside the camera), and the iSRD takes (when it works as it should) very good care of physical scratches and dust on the neg/dia but NOT anything that comes from the camera.
Non of them takes good care of both faults - so why does the scanning process not look like:
Scan of neg/dia
Infrared scan neg/dia
iSRD correction of picture
SRD correction on the already corrected picture
Storing a great corrected picture - Voila, every one is happy
As it is now SilverFast will not produce as good pictures as they could - and possible frustration and bad reputation might appear amongst customers (at least one

Regards
//Håkan