In a previous post, I discussed the Portland Beavers moving to Tucson. Eventually the plan is for the team to move to Escondido, near the major league affiliate in San Diego. When I wrote my paper on the determinants of the locations of minor league baseball teams, I specifically ignored cases where a minor league team is located in the same metropolitan area as a major league team. Partly this choice was because my econometric method did not allow for this possibility, but mostly because this phenomenon did not happen very often. In fact, the league rules pretty much give the major league teams territorial rights to keep minor league teams out of their metropolitan area. The exceptions to these outcomes are either because the minor league team predates the major league team (i.e., the Tacoma Rainiers) or because of the minor league team using creative geography to be just outside the territorial zone (i.e., the Kane County Cougars).
However, in the years since I wrote my paper, the major league teams have decided that either minor league baseball is a complementary good, as opposed to a substitute good, or that having their AAA affiliate nearby is enough of a positive to make up for the substitution effect. The Gwinnett County Braves, the Escondido Beavers and the Lehigh Valley IronPigs all represent examples of teams bringing their AAA affiliate into their metropolitan area, or just outside of it in the case of the IronPigs. Having the AAA affiliate close makes it easier to call up players in case of injury. My guess is that over time the increasing value of proximity of the AAA players and that the changing nature of baseball markets, making minor league and major league baseball more complementary, have both increased the teams' willingness to bring their affiliates into their home markets.