Cat lover speaks...
Yesterday I watched a woman board in C with a small dog in a carrier - I must admit it crossed my mind, "what if people are allergic?"
I'm surprised they allow it too, but I'm grateful because I needed to do it once.
When I moved cross country, I had no choice but to fly with my cat (I could never have boarded her in stowage!) She was quiet the whole trip and it was the only safe way I knew to get her here.