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Kahis made some very good points but I'd add that really an identification is only as good as its accurancy. If you're just mildly curious then you can decide on what lengths you want to go to to reach a good identification because nobody will ever check it and you won't pass it on to anyone else. But if you intend to give the record to a recording scheme then it's your responsibility to make sure the identification is accurate otherwise the scheme will "polluted" with inaccuracy and the maps it produces will not be respected by organisations that need to use the data for local planning decisions.

I think most photo or field identifications (even of common species) also have to be judged less certain than idents made from specimens. Not just because the whole fly couldn't be examined properly but also because it is hard/impossible to double-check them later. Often scientific opinion will change and species will be split or reworked so that it needs even closer examination (eg. genitalia) and if a specimen was taken then it can be reexamined.

It's always possible to study the species that are easy to field-identify but this soon becomes a bit boring and the more curious amongst us was to delv a little deeper and find out what else we have living near us :)