Below is an explanation given in one of our reference guides.
7. MEL PENALTIES VERSUS ECL RESTRICTIONS
The difference between DDG penalties and ECL restrictions for the same failure, can be clarified as follows:
DDG: The requirements in the DDG are, amongst others, based on a (next critical) en-route failure, after dispatching the aircraft with the respective component unserviceable.
ECL: In flight, procedures are based on the assumption that the next critical failure, during that particular flight, will not occur. The manufacturer,s Operations Manual, AOM and ECL procedures are therefore based on different criteria than the DDG.
When consulting the DDG in flight, it should be realized that restrictions in the DDG are only applicable for (the next) departure. Safety of the flight is fully covered by the applicable ECL procedure directives.
Further information can be found in the limitation section and checklist section. I.E. What is the minimum equipment required for cat 2 or 3 or how to deal with multiple failures.
In your example, the wiper failure during take off does not affect cat 3 ops at destination.
Also, one pack failure during flight does not require you to descend to FL 250! Another pack failure would be covered by the NNC, namely an emergency descent.
One gen failure requires the APU(on some AC) failure of the other gen requires a diversion as per checklist instruction section.
Hope this helps
Ps where it says DDG, read MEL. Old term used in our company, since replaced by MEL.