Have you noticed a trend among GMs, statisticians and fans that emphasizes the importance of defense?
Over the past (roughly) five years, the importance of defense has seemingly come back into the limelight. Before that, offense was the only concern of GMs and fans. A guy hits like Giambi? Who cares that he can't field? Barry Bonds can't move? Whatever - he gets on base or homers more than half the time.
Part of that is a greater understanding of the differences between pitching and fielding. What we used to think of as solid pitching (e.g., inducing weakly hit balls) we now think is more a product of solid defense (turning balls in play into outs).
There's also an absence of elite hitters like there were 5-10 years ago (e.g. Bonds, McGwire, Sosa). Teams were willing to put up with their poor defense because their hitting was other-wordly. No such hitters exist nowadays.
Last but not least, the study of baseball has undergone the natural progression that every facet of science experiences. Just as we understand physics better now than 10 years ago, the same is true of baseball.
For example, this study shows that Carlos Gomez (.258/.296/.360 in 2008) was actually more valuable than Jermaine Dye! (.292/.344/.521) - partly due to baserunning, but primarily due to defense, where he saved 32 more runs than Dye.
Gomez should be important for us to look at because of the similarity to Brett Gardner: very fast, very good D, not much bat. Gardner, I'll have you know, was the best defensive outfielder in all of baseball last year (among guys with at least 300 innings). Bear in mind though that he won't be that great over a whole season, but it's extremely promising.
If Gardner can even hit somewhere around .260/.330/.360 (which is perfectly reasonable and better than Gomez), steal some bases and play his usual defense, he will be very valuable.