I believe that athletes, or people who do a lot of physical activity, need plenty of sodium, and there is more of a problem getting enough than getting too much. This is especially true for people who shun processed foods and fast food, which are very high in sodium. The hotter and drier the weather, the more sodium and other electrolytes one needs.
I got dehydrated hiking in hot weather a couple summers ago, when I drank water but didn't get any electrolytes. My body just flushed out water like mad for days, and it took a couple weeks before I felt OK again.
I just had bloodwork done, and all my electrolytes were on the lowest end of normal. I feel much better as soon as I drink something with electolytes, like Surge WOF, Airborne, Emergen-C, etc.