I think it's going to have a lot to do with the manager- and player-selected roster spots, given the 1-player-per-team rule. For example, Cristian Guzman's hitting .330 for the Nats, but Dunn's hitting reasonably well enough that he could theoretically be the Nats' representative, which would leave a potential reserve shortstop spot for Theriot, and MLB can also then use Dunn in the HR Derby. There are more machinations involved for the reserves than simply who's playing the best at each position.