Mills is committed to working in the bench players regularly. Giving Wallace a day off against a lefty is an easy way to accomplish that.
I've always felt instinctively that a lineup where bench players rotate in regularly was the best. I think I got that from growing up watching Earl Weaver's Baltimore teams. He usually had a nine or ten-man pitching staff, and so had enough regular players to have several platoons going on at the same time. For awhile in the early 1980s he regularly rotated three guys in LF -- two righthanders (Gary Roenicke and Benny Ayala) and a lefty (John Lowenstein.)
But some of the best teams ever (The Big Red Machine, 1970s Dodgers, 1960s Tigers) had very set lineups. Those Reds teams had guys on the bench like Jimmy Stewart and Ty Cline, and whatever catcher was backing up Bench that season, who hardly ever played.