Those stains could be there to stay, but if you are using a quality leather cleaner and conditioner, it's still good for the chairs.

I use a leather conditioner that is made for leather car seats. It's put out by Mercedes Benz--I figured that must be a pretty good product.

Leather is simply skin so don't be afraid to get it wet. We've cleaned a lot of leather since daughter is an equestrian. The best thing we've found is Murphy's Oil Soap. Mix it with some water and apply with a rag. Once clean you can condition with Lexol Leather Conditioner.