Since there is no zero crossing with DC like there is with AC current, DC contacts tend to wear until they weld. There should actually be some slight wiping action as the contacts come together. For inductive loads or even commutated loads it is nice to have a set of series contacts to minimize or quench the arc. Twice the contact area on separated contacts will usually result in lifetime contacts. Of course car companies don't do this but it is common in industrial situations. It is also possible to build a snubber circuit if the arc is horrendous.