I have to say I have got little against responsible metal detectorists, I
know one such chap local the myself, and he is very diligent with regard to
recording his finds and does not sell them either.
Interestingly, and I feel I have to agree with him to some extent, he feels
that being a metal detectorist he is involved in the pusuit of archaeology
as much as any archaeologist.
My concern regarding the aforementioned website is more of an issue of
supply and demand.
I quite agree with those who say that you cannot metal detect for certain
artefacts at will, archaeology would be so easy if we knew where everything
was before we dug.
However if the demand is great enough the supply pool may become large
enough for most artefact types to become available.