CDL items don't need a time limit, remember they may have an operational impact for example higher fuel burn but no impact on 'safety' where as MEL items usually have an impact on safety.
By deferring IAW the MEL you are using up the redundancy on the aircraft systems and hence will be operating nearer to the 'edge' as it were. Its not UNSAFE its within the margin of safety (per Part 25.1309) just not as safe... a Systematic Safety assessment by statistical analysis will reveal the limitation that will need to be applied, for example CAT A, B, C or D.
For further reading have a look at the book 'Systematic Safety' which explains how this is achieved; and also why 'flogging' the item off at the end of the time limitation and then deferring the item at the next station because your company has not got the spare can lead you to skating on very thin ice!