Most teams carry back up infielders. Starters need rest days, days off or they get injured or can't meet the demands of baseball's gruelling schedule for a couple of games. I look forward to the national furor over the first MLB player to take maternity leave.
Anyway, having a player that can master any infield position (first, second and third bases, as well as shortstop) is a huge asset. And if that player happens to be a true defensive wizard*, then they add the very useful tactical option of being a defensive replacement. Doing so late in the game reduces the load on the starters and keeps them (hopefully) healthy, while maximizing the team's situational strengths. After all, once you take the lead, the only way to lose it is by giving up runs.
So my ideal utility infielder is a masterful defender who's got the chops to handle any of the four positions. His primary use will be as a defensive substitute. He needs a decent bat, particularly in this line up, for those days where he's spelling a starter.
Selection: Ozzie Smith
Perhaps no other player is as well known for his defensive ability as the Wizard. An incredible athlete with fantastic range, a cannon arm, a fantastic glove and excellent judgement, reasonable estimates place Ozzie's defensive value between 150 and 240 runs prevented above average across his career. Call it ten runs a season better than average, every season for 18 years. That's phenomenally rare. Fangraphs puts his peak defensive year as 1989, when they estimate he saved 32 runs. Even only coming in the 7th inning and later, Ozzie would be worth about 10 runs on defense over an average defender. That's at least an extra win (across an average season) created by just the glove of bench player. I suspect it might be worth more, as Smith would spend his time fielding in higher leverage situations.
While his defense is universally lauded, his offensive contributions are too often overlooked. Admittedly, for his first 6 or so years in the majors, Smith struggled offensively. By 1984, though, Smith had developed into an above average offensive player with a sustained peak between 1984 and 1992. While always lacking for power, Smith managed to combine an excellent batting eye with excellent speed on the base paths (580 SB). Oddly enough for a player of his speed, he did so with a surprising low batting average on balls in play, only breaking .300 three times. Even in his final season, in 1996, Smith was an offensive threat, posting a .282/.358/.370/97 (avg/obp/slg/wRC+).
Across his career, including his offensively anemic Padres tenure, Smith posted a .262/.337/.328/94 line. In his best year with the bat, he hit .285/.380/.367/122. And Smith's ability to get on base better than 35% of the time for pretty much his entire Cardinals career is even more useful talent in the line up of this team.
By many accounts and metrics, Mr. Hoover was the greatest defensive player ever. Fangraphs estimates he saved 294 runs above the average 3B across his career. And he had two seasons above 30 runs saved (1967 and 1968) in an environment where runs were even more precious than they are today. And he had an above average bat. Where Ozzie had speed and a discerning eye, Brooks had some power, hitting 268 homeruns and possessing a career batting line of .267/.322/.401/105. His peak year offensively came in 1964, when he hit .317/.368/.521/145 with 28 homeruns, played his usual excellent defense and won the MVP.
It's worth noting that Robinson's runs above average is calculated against third basemen, rather than all defensive players. While I have no doubt he would've been strong shortstop as well, I don't see him handling it as well a Smith. Or the guy who was playing shortstop along side Robinson much of the time, a guy named...
The other greatest defensive shortstop of all time. Belanger didn't see as much play time in his career as Smith, so his lifetime 241 runs saved is very impressive, but unlike Smith or Robinson, his bat was generally weak (.228/.300/.280/72). Defensively, his best year may have been 1975 with 35.0 runs saved above average (from Fangraphs), making him an All Star worthy shortsop with only a .226/.286/.276/70 batting line. Belanger is your guy when defense is your only priority. Belanger isn't a Hall of Famer, though he and Robinson probably deserve a fraction Jim Palmer's plaque.**
Speaking of Palmer, did he not have the best luck with defenders? He started his career with the greatest defensive pairing of shortstop and third baseman ever behind him and ended it with the (arguably) greatest defender to split his time between shortstop third base. Who is, incidentally, also an alternate:
Frankly, Ripken's offense is slightly overrated. He's definitely the best bat on this list, but not by a wide margin, at least averaged across his career, hitting .276/.340/.447/111. Of course, that was an average across a 20 year career where Ripken played EVERY SINGLE GAME for 16 years. The Iron Man probably traded reliability for peak performance, but his peaks were quite high, most notably his 1991 season, where he hit .323/.374/.566/156 which was also his best year defensively, saving 23 runs above average according to Fangraphs -- arguably the best season ever from a shortsop.
Regardless, neither Ripken's durability nor defense were in any way overrated: 2632 consecutive games and about 180 runs saved above average (Fangraphs again). Ripken is a worthy choice as starter at shortstop (particularly for his peak season) and makes an excellent choice for utility infielder.
*Yes, this is a foreshadowing clue. **For what it's worth, Palmer's ERA is about 80% of what his Fielding Indepedent Pitching (FIP) estimates what it would be with an average defense behind him. And wow, how did Baltimore end up with something like a third of the 10 all time greatest defenders?