Actually, you're right. I had a sort of secondary concern about why someone cared whether it was primary or secondary. Surely the important part was understanding the degree to which it could be relied upon to draw conclusions... I can see the primary/secondary distinction was important when I did history at A level because the concern was to get us to start approaching texts critically and its a useful crude categorisation at that level but once you get beyond that it is presumably more useful to approach each piece of evidence on a case-by-case basis/