I think it’s useful to have other fitness elements in your routine not just riding, though I’ve dropped them all in favor of barn work!
If the seminar is being given by someone who teaches both ballet and riding I would definitely go because there may be interesting observations on rider biomechanics that apply beyond the ballet focus. I went to an excellent clinic a few years ago given by a riding instructor who was also a Pilates and yoga teacher with kinesiology training. There is much less sports mechanics info out there on riding than some other sports and much of the traditional riding instruction blurs what’s really going on with the body.
Though IME the usefulness of dance classes correlates to how much natural ability you might have. For someone like me who is not super flexible or graceful, anything that gets me to stretch and move in rhythm is useful. But the finer points of ballet will be lost on me as I will never get to that point in the curriculum!
Salsafit or zumba was a great fit for me. Also I did bellydance for several years and totally agree with how it unlocks your hips! On the other hand I certainly saw other beginner students who couldn’t do a shimmy isolation because their hips were somehow locked to their torsos. I expect they would have trouble riding. Maybe riding coaches should teach shimmy to their beginner students!
Dancers in the other hand have a natural head start if they go to learn riding.
I imagine that the benefits of ballet will be posture, rhythm, fitness, and awareness of body in space, as well as getting used to dancing to classical music so you can better develop a classy free style soundtrack!