A new camera body should never be seen as a investment. It will always drop in price as time goes on. A investment is something you buy with the expectation that its value will increase.
I don't think he is looking at it like a financial investment as in the body going up in value. I think he's looking at the additional high ISO capabilities making f/4 lenses an acceptable speed (since DOF doesn't matter to him) where they would normally not be. An extra $1k on a body is less in the long run than getting all your lenses one stop faster.