See Wikipedia (and numerous studies done on this): http://en.wikipedia.org/wiki/Electrolyte

The sports drink craze really is just a marketing ploy (almost as bad as “vitamin water”).

After we have our exercise, we need some water, but drinking too much plain water will dissolve more electrolytes and electrolytes concentration in our body will decrease. This will promote other bad consequences such as nervous system disorder, as electrolytes play a big role in transmitting electric impulse in our body.