I will always have my primary team in a sport and will root for them against anybody else. But, I can't get into a game if I don't care who wins, so I tend to hop on and off secondary bandwagons. In any given sport, I usually like a couple of the top teams and don't like a couple other of the top teams, and those teams may not be consistent within eras (for instance, my not liking the current Warriors wouldn't mean that I couldn't pull for a version of the Warriors in 20 years). But, my actual team I am a die-hard fan of is something I keep constant.
Yeah, I see what you mean. My only "die hard" loyalties are in European sports (Go PSG (my soccer club) and go Stade Français! (my rugby club)), so in the US I don't really have "a team" (except for LSU in college sports, but I don't follow college sports much), but in the pro sports there are teams I like and teams I don't like and I'll generally have somebody to pull for, and my team choice is generally due to either personal links to a place or due to finding a place attractive. If I move to the US I'll surely end up pulling for the local teams in the area I move to.