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Well, yes and no :) Depends on how and why you study them :)

For almost all flies you need a collected specimens if you want to be really sure of the identification. *Good* photographs are often enough, but very few photographers know exactly what details must be visible for each family/genus/species.

Nearly all diptera identification books are written by taxonomists for taxonomists; they care little of field identification. In fact most species are probably identifiable in the field, but nobody has the skills needed.

Personally I don't think twice of collecting a fly. After all, driving to the study site on a hot day probably kllls more insects than collecting there. But I try to kill the collected flies as quickly and kindly as possible. In my opinion recording species *and* publishing the records on the net & reporting to authorities responsible for nature protection makes collecting morally acceptable.