The use of an MEL to give Defect Relief is to acknowledge the removal of a level of safety or reliability from the effected system in your aircraft.
This level of safety or reliability puts (exponentially) a greater safety or reliability risk to the remaining levels of redundancy.
The times quoted in the MEL/MMEL are not plucked from thin air based on the feeling of pilots, but actually worked out using MSG-3 techniques and reliability reports from your airlines/operators, and given the best reliability time that should be tolerated by the remaining levels of redundancy.
Extensions to MEL's should be avoided, however; extensions, consulting the individual aircraft or system's defect reports going back some pre-determined time, may be granted by your AMO or CAM.