Mike, keep in mind that most beers require a pure yeast culture for fermentation. This is because there are lots of unwanted off flavors that can be created by wild yeast and bacteria. This is desired in sourdough and "wild" beers, but would be definitely distracting from the taste of a lager, for example.
because of that and the inevitable contamination that happens with constant reuse of yeast, you will not be able to keep your yeast culture going. Unless you get into yeast ranching and sterile yeast storage/propagation techniques you'll have to buy a fresh culture of yeast at least one in a while.