The Yankees have made no bones about it: they like speed in their outfield. They've demonstrated so by signing Jacoby Ellsbury to a seven-year contract in early-December and then proceeded to sign Brett Gardner to a four-year extension a few months later. The speed these two provide won't just benefit the team defensively in the outfield, but it will certainly help the team on the base paths (in terms of stolen bases) as well; we just don't know how much exactly.
Jacoby Ellsbury stole a total of 52 bases (most in MLB) in 56 attempts (second-most in MLB) last season for the Red Sox. All told, Ellsbury had 267 total opportunities to steal a base, and he attempted to do so about 21% of the time (league average is 5.5%). That 21% stolen base attempt rate, however, isn't quite as high as some of the more aggressive base runners (Starling Marte: 56 attempts, 26% SB attempt rate; Jean Segura: 57 attempts, 26% SB attempt rate), but is still a very respectable clip. Besides, Ellsbury has been very efficient on the base paths, not just last year (93%), but throughout his career as well (84%); he has a pretty good idea of what he's doing out there. At the same time, though, I wonder if Ellsbury will try to run a little more this year given Derek Jeter's double play tendencies, but we'll see.
In comparison to Ellsbury, Brett Gardner stole only 24 bases (15th-most in baseball) and attempted to do so 32 times (20th-most in baseball). Gardner did have 224 opportunities to steal, but only did so 14% of the time. Gardner's seemingly passive approach on the base paths last season left fans up in arms and we never really knew exactly why he didn't run more often. Maybe because the Yankees didn't want to risk him getting caught, they could have given Gardner the red light more often so Robinson Cano, the team's only real power threat for a large portion of the year, could drive Gardner home. Or, there could have been some sort of injury we didn't know about and that could have been a reason why Gardner didn't attempt to steal more bases.
Sure enough, in an interview with Michael Kay a few weeks ago, Gardner did say that he dealt with some "dead legs" and "soreness" at times last year and that he had to play more often given the team's rash of injuries. Now that the Yankees' outfield is stacked this season, Gardner should be able to get a breather every now and then so he can be a little fresher as the year goes on. In Gardner's last full season prior to 2013 (2011), he attempted to steal a whopping 26% of the time (62 attempts, 240 opportunities). I'm not sure we'll see that kind of aggressiveness out of Gardner this year, but hopefully he'll come somewhere moderately close to that level.
For what it's worth, Ichiro Suzuki, the Yankees' third stolen base threat, attempted to steal in just 10% of his total opportunities (24 attempts, 231 opportunities) last year. This is down from his 15% stolen base attempt rate from 2011-2012, and, given he was 39 years old last year, it shouldn't come as much of a surprise that he has run less. Now that the Yankees have a combination of Ellsbury, Gardner, Carlos Beltran, and Alfonso Soriano in their outfield, Ichiro is set to be the team's fifth outfielder and should be used as a pinch-runner late in games. If so, I would like to see Ichiro improve his stolen base attempt rate from last year's 10% clip to, perhaps, his 15% rate from 2011-2012. That shouldn't be asking for too much since his legs will be fresh from spending more time on the bench.
Sure, there are players like Ichiro (and perhaps even Soriano) who can steal a bag here and there, but Ellsbury and Gardner are the headliners when it comes to stolen bases. Ellsbury and Gardner are important not just for their ability to play strong defense in the outfield and get on base, but for their ability to steal bases; those two being aggressive (and having good success) on the base paths could go a long way towards the Yankees getting back to the postseason.