What does the "Scavenging" stat do? How useful is it?

It actually is something like +x % for Magic find. It increases your Chance to loot better gear, techs etc. We compared in a group the various scavenger percentages. Overall, the higher, the better. Its really nice to get yellow tech, when you only get a green one, if you didnt give some % into scavenging.

Some Addition: Scavenging will not increase loot count. But it will increase the Chance, that your loot is of better quality (yellow instead of purple item, not 7% crit instead of 5,5%)