Have levels of privileges
In some cases, usefulness does not apply as criteria for monitoring internet activity.
From limited access upto unrestricted access.
EDIT: Say, Junior-most developers/trainess have no access and no file download rights.
One level up, maybe access to forums and QA sites related to field and/or restricted download bandwidth.
Another level up, no download restrictions but site monitoring.
And even higher, you can decide how much you want to relax for the top management.(most probably unrestricted)
If it were upto me I'd only put restrictions on file download rights. Site monitoring is somewhat self-imposed by the workplace environment, I feel.(Or you can even take a signed undertaking for such things and deal with cases that get extremely out of hand.)
I wouldn't waste time on that. But watching my bandwidth? Yes.