I overstated Nava's baserunning deficiencies. I'm probably biased based off that terrible game he had a few weeks back. He won't add anything on the bases, either. However, I think the baserunning issue is becoming a red herring. I'm more alarmed at Nava's terrible defense in the field, and his inability to his left-handers thus far. Again, the disagreement we have over Nava is his potential competence as an everyday LF. I have no problem playing Nava in LF against right handers. He has some value in a platoon. Together, he and Gomes could represent a 2 win player in LF. Not great, but not terrible either. I'm just of the opinion that if you play him everyday, he's going to give more runs back in the field, and he's going to see his numbers decline because he struggles against lefties.
Getting back to baserunning, I think you are dismissing the value that good baserunners can bring to a team. Home runs and offense are down from the historic levels during the Steroid Era. That means that the stolen base is becoming a more valuable weapon. Ellsbury and Victorino are well-above the break even rate. Without question they add more on the bases than they take away. You also have to factor in their ability to take the extra base. They aren't hugely important, but there is still value to be had in this aspect.