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'Us dipterists' idea' meaning Hennig's? ;) It is certainly a criterion but at times when evolution is hypothesized to have been in many consecutive splits with small groups then we either end up with an enormous multitude of groups at a certain level or we get a very large inclusive group that harbours many different well-recognizable groups that have been in existences for a long time (like McAlpine's proposal to lump Heleomyzidae, Sphaeroceridae and a number of smaller families into Heteromyzidae).

Sometimes it is also good to relaise that what may have seemd a good idea may not be one in all circumstances. Is it worth our while to disrupt the whole classification within orders or families because we want to have our hypothesis of evolution reflected in the classification at all times? Something we can have a nice, long discussion about. :D