The idea of the mod is, because the way AP have designed the cover, the std pedal mechanism will not disengage the clutch fully even with the pedal all the way to the floor. The std ratchet auto adjustment is a pain, so even on a srd clutch u could use this mod to get rid of it. On an AP clutch u have no choice, u have to go manual. The mk5 quadrant is cut down so that you have enough movenment, and the the pedal doen't sit too high.

A stretched cable won't have much bearing - as u can manually adjust to compensate, so the clutch bites at a comfortable position.

Replacing is done for 2 reasons. First, if it's old, then a new cable will give a more fluid movement due to less friction internally. Secondly, it's working a lot harder with a heavier cover, it's under more stress full stop. I had one snap a few months ago - it lasted 4 years and i do a lot of traffic driving so it does get used a lot. I know other people who have had them snap too. For the price of the cable, i think it's a worthwhile precaution.