Karl also makes this intersting observation in his "Highway to Hell" article cited below:
But what are Austrian metal detectorists really digging up? Do they typically dig sizeable trenches, and do they dig down into stratified contexts? As far as can be ascertained from the results of my survey, they normally do neither. Rather, the overwhelming majority restrict their activities mostly to digging just the topsoil (Figure 10) and to digging pits of less than one-quarter of a square metre (Figure 11).
Yet, the topsoil is that part of an archaeological stratigraphy that is usually removed by a mechanical digger on the vast majority of Austrian excavations. This is true for pretty much all rescue excavations, and even for many, if not most, research digs. Manual removal of the topsoil is the rare exception to the rule, and even where this happens, the topsoil is rarely thoroughly searched for finds (least of all using a metal detector), if at all.
Thus, most of the activity of metal detectorists seems to be limited to those parts of archaeological stratigraphies neither observed nor documented in systematic archaeological excavations. It thus seems rather peculiar that we accuse these amateur archaeologists of intentionally destroying the archaeological contexts of their finds. After all, professional archaeologists rarely even bother attempting to recover the finds that derive from topsoil contexts; rather, they run them over with a large digger or remove them rapidly and with little regard for implementing intensive recovery strategies.
See more here: http://ordinarymag.blogspot.com/2012/01/on-highway-to-hell.html