Good question.
Most of the generic domains I bought I thought hard about before registering, so I almost automatically renew them, whatever traffic they have.
But I reckon if a slightly doubtful name is getting 30+ hits per month, there must be something inherrantly useful about the name and it could well have some sale value.
For those who are only interested in income from traffic (not sales), it will depend a lot on the quality of the traffic and how much money the domain is earning, not just the number of visits.