I also don't like fans that support both teams in an area. This is actually a minor problem in the Bay Area, where you see punk-asses wearing stuff like this (to be fair, I haven't seen one of these in awhile):
I'm sorry, you're either a Giants fan or an A's fan! Pick one and stick with it. You wouldn't see a Mets/Yankees cap in New York, would you?
For transplants, I totally respect it. If you're a real Yankees fan from New York who is living in the Bay now, you rock that Yankees shit. I still don't like your team, but you're a real fan.