The reason we have a Yankees forum is very simple: To post Yankees stuff, and for Yankees fans on this board to discuss Yankees stuff.
The discussion about why so many Yankees fans hang out here is a completely separate one (though I imagine it is at least partly due to the severe mental disabilities inherent in Yankee fandom; can we really expect rational decision-making from these people?!). They're here, and they're not going anywhere. The alternative to having a Yankees subforum is having all Yankees threads in the General Baseball forum, which would make that forum twice as large and much more unwieldy to navigate. Alternatively, the Yankees threads could end up being posted as comparison or contrast pieces to the Red Sox, and end up in the Sox forum, which is horrifying to consider. Imagine all of the threads currently active in the DY forum in the TalkSox forum instead, just with a few lines added about how it makes them better than the Red Sox.
The Yankees fans on this site (well, most of them) are here because this is a great site for baseball discussion. Go to the YES forums, or even some of the official Red Sox fan forums. They're mostly garbage. In between the spam and the membership counts in quadruple digits, you have far more unrestrained trolls and an amount of useless off-topic discussion that dwarfs any actual insightful conversation about baseball. Jacko contributes a lot to this board, and when it does not directly involve the Yankees, his ideas and statements are almost guaranteed to be part of a great conversation.
We may fight, and we may have rivalries and occasional personal attacks, but that's par for the course on message boards. I'm pretty sure, at this very moment, someone on a message board dedicated to Knitting and Crocheting For Seniors just called someone else a ****waffle. But by and large, at the end of the day, this is a much better place to talk about baseball (the reason we are ostensibly here) than about 98% of baseball websites.
Yankees fans suck. But the Yankees fans on this site, suck though they might, are our Yankees fans. You don't kick your pet out of the house or put it to sleep just because its nonsensical yapping and frequent incontinence might inconvenience you sometimes, because deep down, you love your little idiot.
Finally, if none of the above arguments make sense to you, just remember: the Yankees and the Sox have a rivalry that goes all the way back to a time when people wore bowler hats unironically. It's the most well-known rivalry in sports. People who know nothing else about baseball know that the Red Sox and the Yankees are bitter enemies. I bet most of the world knows. I bet if you asked the Pope, he'd say "Who are you and how did you get in here? Security!" That kind of rivalry virtually guarantees that we'd be talking about all of this stuff anyways. Keeping track of Yankees games, prospects, and roster moves is a big part of the life of most die-hard Sox fans. It just so happens that Yankees fans are starting these threads, but we'd circle around to them in our own time even without them.
What would our lives be without Yankees fans on this board to ******** with?