It is less and less common I think, as kids today (kids today!) grow up with access to any team they want, or whatever star player catches their fancy. I have a 14 year-old nephew in the Lehigh Valley and his favorite teams are the Phillies, Devils and Cowboys (!!!). His Dad isn't into sports so that probably made it easier for him to pick his own too.
It's also most common in the South and West. 20-25% road fan attendance not uncommon for some sports/markets.. And however loud the Phillies phans were in Cincinnati last night I'm sure the Pirates and Cardinals and Indians fans are even louder, just like we always represent (and vice-versa) in DC and Queens.
But I've always been fascinated by the process - I have friends from Philly whose kids root for the teams where they live (and in some cases the parents just go with it at that point). And also adult friends who have given up their hometown team for their current city's team (including Cubs and even - amazingly - Red Sox - fans - who are now Yankees fans). I have never been able to do it, even when I've had season tickets to a local team and they won a championship. It was fun, but still not the same.