Potential also means possible. Pillar/ whatever do not have the potential to be contributing baseball players and provide wins for this team. If Stroman under-performs or gets injured and Dickey starts acting 40 we still have guys like Estrada, Nolin, Norris, Sanchez and Graveman who can come in and provide one or two wins if not more (Norris/Sanchez). If we spend money on pitching and leave LF with a Pillar platoon it will be an error in priorities.
With Melky or Aoki in LF, this teams offense and defense #Martin are good enough to overcome a slightly below average rotation.