Gentlemen (and Gentlewomen)
Without getting all technical and type specific, I think it is fair to say that legally the MEL only applies before departure (definitions may vary).
However, it is our job to make use of all available resources, of which the MEL is one, including in flight.
We then need to make an informed decision as to the safest course of action.
I will also say (although it should be obvious), that just because the MEL allows us to depart with a certain item inop, it is still the commander's responsibility to accept (or not) the aircraft. This becomes especially applicable in a multiple failures scenario.
Bottom line: I would always try to look at the MEL for further guidance when deciding on the safest course of action, on the ground, or airborne. However, the need for this will greatly depend on the quality of the abnormal and emergency checklists provided. On some aircraft they really don't go into a lot of detail so any additional info is most welcome.