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The problem with the use of 'field guides' (even with hoverflies) is that an inexperienced user is tempted to choose a picture that looks most like his/her insect and think the job is done. But a more experienced entomologist knows that often there is quite a large amount of variability and that is why keys use the most stable physical features - most of which are not clear on most photos. This is especially true if the photos are taken for aesthetic reasons because they are likely to show-off the insects most attractive colours and not the features needed to identify them (eg. legs, bristles, wing venation etc).

Also, it really depends how accurate the photographer wants to be. For example, given a large hairy fly, some are happy knowing it is a sarcophagid ... others might not be happy until they know it is Sarcophaga subvicinia, which definitely couldn't be achieved through a simple photo.