He plays all three OF positions which is something the rest can't do. I also think the bat you're seeing now is what it is and it's a below league average bat, yes his D and base running make up for it but if you can get a 2.0 fWAR player on the bench as your 4th OF it might be the smarter way to go. Pompey's complete package is a whole lot more appealing to me than Pillar is.
You can have him in 4+ games a week rotating through the line up, giving Saunders and Revere days off against LHP while still being insurance for Pompey again next season. I like Revere in a line up of guys who strike out as someone you can throw in the 9th slot and figure you'll get a decent at bat out of. If Saunders is healthy it wouldn't hurt to have another LHB that can potentially give you somewhere between 108-126 wRC+ (if the knee is healthy and healed up). It also gets Bautista out of the OF and moves him to a position where he might be less of a defensive liability.
I think that a line up of (potentially)
Travis 2B, Donaldson 3B, Bautista 1B, EE DH, Tulo SS, Martin C, Saunders RF, Pompey CF, Revere LF looks a bit better than slotting Pillar into the 8th slot and running Saunders off the bench. From what AA said earlier this season Saunders is still in the plans for next year and I think Gibbons like Revere enough to play him everyday next season.