For what it's worth, people with good Scavenging skills are finding WAY, WAY more items just out in the middle of a field than I, with a low Scavenge, find in a Junk Pile. And it tends to be more useful items than the appropriately named "Junk" that comes out of the piles (that may or may not get a future need).

The only difference is that I almost never find anything while Scavenging while I at least find something when collecting off a Junk Pile. So the junk piles are useful for people to, for sure, find something. But don't seem entirely required to find materials. Not sure if the possibility of finding significant discrete items exists outside of junk piles though -- entire whole tools, the fabled "old saw blade", etc.