You won't improve on badly recorded stuff. Though you may recapture the full quality of the master tape, you will also recapture its defects. You will get consistency - CDs of a particular program will always be the same in quality. As well as picking up dirt and therefore noise as they are used, cassettes stretch and slightly distort the sound with use. OK, cassettes vary greatly in quality, but no spoken voice cassettes are recorded on metal tape, or chrome tape - not even on high grade tape in most cases.
Convenience is the main reason for using CD, sound quality is an added bonus.
Main Street was always intended for CD, and extra care was taken over the master tape quality. For one reason or another, English teaching programs were always recorded in mono. One reason was that most classroom playback machines were mono. I noticed years ago that most schools were replacing their dedicated classroom mono players with stereo machines. Grapevine was probably the first stereo English course, and there were sound pedagogic reasons for this.