Thats interesting that you hips never cracked 'til you took Ballet. I'm the same. Never cracked until I took up TaeKwon-Do. I think the cracking comes with increased mobility. I dont think it's bad for you - i think that it gives you arthritis etc (I've looked at tonnes of websites, and they all agree), thats just something people say to try get you to stop. Its like saying sitting in front of the TV gives you square eyes or playing with yourself makes you go blind haha.
Old wives tales.

I love martial arts, and the flexibility that goes along with it/is required.
I've been punched and kicked hundreds of times in various places on my body and I 100% believe that none of those blows is the reason why a some of my joints crack. My joints crack because I'm flexible. It's kinda like brushing off noisy cobwebs when I stretch, awakening my body. The cracking is not sore, and in fact once I have cracked I can stretch further/my range of motion increases.

I think DJ is winding everyone up - I dont think he's got a sore neck at all , I think its some guy making it all up. it might just be somebody getting his kicks.