As I understand it, it won't. I only looked at the expanded DR rules once or twice (having not upgraded since 3.0), but as I understand it, there are at least four types of DR: Silver for dealing with traditionally Unholy monsters like Vampires and Werewolves, Cold Iron for dealing with Fae, Magic for dealing with powerful supernatural beings like Dragons, and Adamant for dealing with insanely durable beings like top level Monks. Silver works because it is Pure, and the weapon needs to be at least largely pure silver to do the job. Cold Iron works because it is a natural conduit of the Earth Element or some such, and again needs to be pure to do the job. Magic..... can be applied to any of these and still work, but must be adequately enchanted to be useful. And adamant needs to be pure to have the raw strength to overcome the pure physical toughness of the target. So, unless you are planing on carrying an exotic double bladed weapon, where one blade is one material and the second is another, with the whole thing enchanted out the wazoo, you will need at least three primary weapons.
I suppose one COULD have a spear with exchangeable heads, and each head function as a dagger when used separately, to get around the whole mess, leaving them with either one spear and two daggers OR one staff and three daggers as their melee weapons, but even then anything more then a sling for a ranged weapon would be getting cumbersome....