I understand your concern Cyril. This why I try to find an open source software and musescore has all the potential to be one of the best scoring softwares, but they are too stubborn in this position... They are not "open minded". They didn't paid attention or even tried to do what some members recommended... adding VST or AU Plugins. Maybe they don't have the right personal do it right.
External libraries which have indeed, a better and realistic sound. The FluidSynth seems too amateur-ish, I think it is rubbish compared to most external libraries out there like edirol or GPO4. Even Edirol and GPO4 is better than that FluidSynth system they use. But like I said, it is up to them to decide.
Ricardo