I've found that I have done the most growing as a photographer by shooting subjects that are outside my comfort zone, or subjects that I would not have sought out on my own.
Otherwise, I don't really know how I feel about this subject. I guess I wouldn't turn down a shoot request from someone in a cosplay that doesn't interest me, but I wouldn't actively pursue them either.
I don't think being "biased" toward friends is really biased at all. That's just human nature. As it relates to photography, I always get better results the second or third time I shoot someone - we're more comfortable working together, and I start to understand the nuances of their face, body, and demeanor, so we can create the most flattering poses and lighting setup.