Allowing bulk spam mail to use that channel of contact is contrary to its purpose. It's not a place where I get high priority messages with delivery confirmation to make sure I get them anymore. It's a place where random flyers get stuffed, and the government insists on mixing their messages into that stream of crap.
USPS should stop bulk delivery entirely and have FedEx and UPS do it. They're not allowed to put stuff in my mailbox (and literally can't, because they don't have keys). At least then we restore the line. Things in the mailbox are important. Things that get left on my doorstep might be, but probably aren't.