That's actually important. If maintaining the letters requires some nonzero effort, they'll want to weigh maintaining letters with other things they could be do

That's actually important. If maintaining the letters requires some nonzero effort, they'll want to weigh maintaining letters with other things they could be doing with their budget. I'm not sure if they have to be reapproved by a judge each time. If that's the case, then on average it seems like you'd have greater churn on the letters. And even if there's a near-100% approval rate while investigating, it seems harder to argue to a judge as the years go on. It won't help if Edward Snowden was the target of your letters, of course.