Are FBLs required for a clean mail stream?

A few years ago I would have said that a good mailer could have a good mailing program without necessarily participating in FBL programs. I’m not convinced that’s true any longer. As the mailbox providers and ISPs develop more complex filtering methodologies, it’s important for senders to get any possible feedback from recipients. That press on the this-is-spam button may not actually mean the mail is spam, but it does mean that recipient really didn’t like the message.
Getting the feedback lets a sender fine tune their sending processes and better target what their recipients want to receive.
I do think that senders need to know what users are saying about their email. When users hit the T-i-S button then that is valuable information about how the recipients think about the mail. Senders really on top of things can use positive data (opens and clicks) and negative data (FBLs and unsubscribes) to monitor how wanted their email is and make adjustments to their sending stream.