Another reason why Mercury may be interesting to Earthlings
I have long suspected that Mercury may contain higher concentrations of heavier materials in its crust. Note I said MAY because there are a lot of geological factors that might be put into play here.
First, Mercury being closer to the sun is likely to have its lighter materials blown away during planet formation. As such, inner planets are rocky, outer planets are gaseous. Not always, but that's an observable pattern, right?
Second, Mercury may have remained volcanic for a longer period of time. This is both good and bad. Kimberlite pipes contain diamonds but were once (as I understand it) magma tubes from the mantle to the surface. Other 'heavy' things can also end up in (or near) magma/volcanic pipes as I understand it, including gold [though the mechanism for it getting there is a bit different]. If Mercury remained volcanic for longer, there may be a LOT more of these with accessible minerals in them (more than on earth). And if that is the case, the volcanic structures could be indicators of where to start digging...
Alternately, with a molten surface that lasts longer than on Earth, all of the heavy elements could have sunk to the center of the core, giving you LESS of it at the surface where we might actually reach it.
Still, I would bet that at the poles you'd have many areas that never see sun, others that only see a fraction of sunlight, and everywhere else, 100+ effective mining days available for robots to locate and extract things and launch them back to Earth. It may become the geological find of the millenium.