so it's hard to imagine that outbreeding could be so valuable

I suspect that my characteristics are partly due to hybrid vigor, as are my sister's. It's not so much "neg (inbreeding depression)" but more of a "when pooling both the strengths and weaknesses of two respective gene pools, the strengths of having access to more 'highly beneficial' genes tend to outweigh the weaknesses of being struck with 'detrimental genes' from a different region". So hybrid vigor isn't necessarily antisymmetrical (I doubt it is) to inbreeding depression. Beneficial/detrimental being relative to the current stage of human civilization in general, and your culture in particular, of course. Example: propensity for thalassemia, good once, bad now.