Suppose a tag suddenly begins to traffic important messages, and I want all messages forwarded. One example would be using job.z.acct@xoxy.net to apply for jobs. Once I have had held a job for a few weeks, mail to that address becomes useless, so I let the blackhole eat the junk. But when between jobs, I want that address to forward everything from everyone.
I have tried setting the exclusive sender to "*" to pass everything through, but it doesn't work.
Job searching is just one example. There are other cases where a disposable address becomes more important than I had originally envisioned, and I want everything to be forwarded.
Sometimes I even know in advance that the recipient (or group thereof) of an address is trustworthy enough not to subject their flow to a counter/blackhole, but I still want them to have a custom address without opening a seperate email account. In these cases the best solution is to make the most popular sender exclusive, and then periodically monitor and refill that record.