Repko is a superior outfielder to Span in every way. Speed, catching, and arm are all better. So then why doesn't Gardy start Span in a corner outfield spot, and Repko in CF when both are playing?
The reason is simple. Playing in CF in Gardy's world means you need to bat leadoff, right before whomever the second baseman might be that night. And despite Span's recent mediocrity, he is a better choice then the slumping Repko as the leadoff man.
Maintaining the sanctity of the batting order is more important to Gardy than properly utilizing his defensive assets.