The letters are going out whether you like what they say or not, at least under the system with committee letters. Interfolio might be different, I don't know how it works. The whole point of confidential letters is to encourage candid response, so, yes, there's a benefit in saying "I'm going to sign away my rights to see them because I feel confident that I've been able to find 5 people who respect and support me 100%". The only benefit in knowing what is in them is if they are bad and you need to do damage control. And if that's the case then I suspect you've got bigger problems anyhow.
And BTW, the waiver I signed had nothing to do with the family rights act. It was explicitly talking about the letters my letter writers were being asked for, and it was right on the letter request form, so they know right off the bat that I've waived the right to see it. It's a pretty common thing, but again, I don't know if things are different when you don't use a committee.
The money you pay into this process doesn't entitle you to much. Get used to it.