Well, I have been belly dancing for 11 years, and I just started taking a beginning ballet class. I love it, and plan to continue. I had no previous background in ballet or other western dance forms.
What I am getting from the class is:
1) improved balance, especially in demipoint, which I use heavily in belly dance.
2) lots of practice doing various kinds of spins, which is useful in belly dance
3) general improvement in lower body strength and flexibility, and also endurance.
4) a general refinement and elegance to my posture and carriage.
5) a general ego boost that at 42, I can make it through the class alive and without injuries.
I do not feel like I am in any danger of losing hip or torso flexibility, or becoming "too balletic" in my movement. Let's face reality -- I'm 42 and its a beginning ballet class.

