FedEx had this idea many years ago. They actually even ordered (and received) 737 combi's so they could convert them to pax hauling birds during the day (when they would otherwise have sat idle). They eventually decided (before the actual start of operations, I believe?) that there was no money in the pax business and that flying people during the day would hurt their night time reliability. In retrospect ... seems VERY insightful. (Reference for the above: Prof. Sigafoos's book, link below)

Amazon.com: Absolutely Positively Overnight: The Unofficial Corporate History of Federal Express (9780918518682): Robert Alan Sigafoos: Books

Also ... didn't UPS try the passenger business "thing" for a short while?