sterling was used for coinage for years I can't see it being so soft to render it useless. Various common high content silver alloys can have a hardness of 3-7. Now a hardness of 3-4 isn't very good for weapons, but 5-7 certainly would be. Compare to steel which has varying hardnesses from 5.5-8 depending on the alloy. native silver has a hardness of 2.5 so yes its very soft. But keep in mind that a razor sharp edge isn't necessary for hacking away at something; I've got a machete here, not the strongest iron (It can bend rather easily), and the blade is dull as a butterknife, but with enough strength behind it, and a direct (not angular) swing, it can smash through a 3" round piece of fresh wood with one blow and can do this repeatedly. the effect on a watermelon is even worse, literally explodes the thing, not a clean cut, but a nasty one and I reckon this to mimic the kind of damage it can do to a head. So a silver weapon is impractical, still can be quite deadly and effective. Perhaps a silver sword isn't best suited as a long sword, but rather a broad sword, better for a crushing blow than a razor edge.