I will admit VX had a good point about maintaining loyalty to a team vis-à-vis the lack of loyalty players show to teams and teams show to cities, and I generally don't take issue with the concept of cherry-picking, given my own team affiliations (mostly derived from family and friends as a youngin').
Where I take issue is people cherry picking from the historically successful franchises of their respective sports. It's a lot different when you're the fan of a historically bad/unlucky franchise because, really, who would really claim to be a fan of said franchise if they weren't? I'm not going to question, say, an Arizona Cardinals fan in North Dakota... but if you're cherry picking good franchises all over the map (as one person I knew who claimed fandom of the Yankees, Celtics, Cowboys and Canadiens), you're gonna come off as a tool, especially if the franchise success is recent and the fandom was never notable before said success. A good example of this is the exponential explosion of the Yankees fanbase outside the Tri-state area over the last decade.