I disagree. If your goal is to collect metals only for the purposes of returning to earth, then perhaps you are right in the short term. However, if those materials are used off-planet (or on-site) for the further production, it makes a LOT of sense. Once the foundation of production is in place, mining and building <more spacecraft, Mars habitats, ...> becomes almost exponentially more efficient to do off of Earth (especially factoring in time requirements). Beyond that, there is a good chance that future technologies will require 'rare-earth' and other precious metals that exceed quantities that could ever possibly be extracted here at any cost.