When healthy, Mark Ellis has been an everyday second baseman his entire career with the A's and the Dodgers. He has a reputation for very good defense with a decent bat. With a career slash line of .265/.330/.390, a wRC+ of 95, and 77 runs above average according to UZR, his reputation is pretty spot on. Despite entering the season at age 36, Ellis is probably better than you think.
Take a look at the following stat lines:
Player A: 448 PA, .303/.358/.447, 123 wRC+
Player B: 724 PA, .301/.372/.415, 115 wRC+
Player C: 541 PA, .300/.349/.439, 117 wRC+
Players A and B represent the 2013 stat lines of Jhonny Peralta and Dustin Pedroia, respectively. The stat line for Player C is Mark Ellis against lefthanders from 2010 through 2013. Because of his defense and the lack of platoons for middle infielders, Ellis' success against lefthanders has been masked by his struggles against righties. During the same time period, Ellis has hit just .254/.316/.329 for a wRC+ of 80.
Ellis is unlikely to be in a straight platoon, but he is likely to receive almost all of his at bats against lefthanders where he has excelled. Although he is in the decline phase of his career, his ability to hit lefties has not fallen off in recent years. As seen in the chart below, only once in the past four seasons has Ellis not been an above average hitter against lefties.
When Ellis signed, many of the positive attributes were discussed: solid glove, insurance for Wong, potential platoon, good clubhouse guy. Perhaps the least discussed impact that Ellis will have is as pinch hitter. Shane Robinson was often the Cardinals' best right handed bat on the bench last season. Ellis provides a legitimate upgrade off the bench as a hitter. Earning $5.25 million this season, Ellis serves as both a luxury and an insurance policy.
With Kolten Wong's ascendency, Ellis's playing time will be limited. Given the organization's investment in Wong, they are likely to give him a solid amount of at bats against lefties to continue his development as an everyday player. Ellis may be limited to 100 at bats or so subbing for Wong and pinch hitting, but given Ellis' history they are likely to be high impact at bats. If Ellis' rate stats approach career highs at the end of the season it will be because his strengths are being optimized, not because he suddenly got better at age 36.