Apparantly the weekday/Saturday free discs add around 100,000 purchasers of the paper and the Sunday adds up to 600,000 extra copies sold (Prince CD the best 'sellar'). Discs are produced months in advance during the Summer when the productions lines are quieter (thus around 0.16 pence per disc).
Sunday discs will continue as readership on Sundays is not terribly loyal anyhow and the extra sales here really matter. The Daily Mail management feel that as a long-term strategy the free discs do not work as all the additional sales are for the givaway days only and the free discs are expensive to produce. No mention of the paper not being good enough to retain new readers
I do like some of the free discs but environmentally producing so much stuff that no one wants badly enough to actually buy is frightening.