The decision should never be one taken by one person (group) in isolation. Fitness to fly is the end product of the efforts of a number of groups -
(a) the OEM/regulatory design engineers, flight test personnel, certification folk and ICA groups .. etc
(b) the MMEL/MEL stakeholders
(c) the operator's tech services group, maintainers and aircrew/cabin crew
In respect of maintainers and aircrew, the maintainer (and this includes a pilot exercising an MA away from home base) has to release the aircraft before the pilot gets to consider the matter. The pilot, then, is the final link in the safety chain and ought not to just accept a snag deferral unless he/she has assessed all the operational matters which might impinge on the particular snag for this particular flight ... on most occasions, the MEL write-off is accepted and the flight proceeds. On some occasions, however, the pilot will decline the flight on the basis of some rational operational assessment.