It did cross my mind that the players union itself may object to the idea of piggybacking. Considering arbitration raises, not to mention free agency contracts, still largely depend on old school counting stats like wins, Ks, innings pitched and so on, it would stand to reason that pitchers in a piggyback rotation are simply not going to accumulate those stats at the same rate a pitcher participating in a traditional rotation would.
Not that the Brewers dip into the starting pitching free agent market often, but it wouldn't surprise me if a starter they were after had concerns knowing that he wasn't going to get the opportunity to go 5+ innings every five-six days.