Suggest you run a search as this topic comes up in PPRuNe from time to time.
Considerations -
(a) pre-despatch U/S has to be considered from a certification point of view .. hence limitations to preserve a notionally constant risk level with the U/S as an intentional operational matter
(b) in-flight, just one of those things which happen along the way. However, once you have done the QRH things, it is a good idea to check the MEL as that gives you some insight into other considerations which you might not have thought about .. if you are able to comply, there is no reason why you ought not .. and, in so doing, you are reducing those risks which the MEL addresses.