It's not an exploit switching armor to 3 of the same class. People do it cause the loot system is insanely rng. They do it so they have a better chance to get a full set of raid great for said class. And as for the strange coins, the vault is shared. Mats, motes coins, and ammo can all be shared. Xur is a 2 day thing once a week. They bound what currency to obtain rep gear which is the major end game currency. Strange coins are rewards for your characters to spend on special gear and upgrades. It is not reputation sensitive.