I agree with Tiny that too many activities aren't good for a child so young. You need to be bored to stimulate your imagination, and learning to amuse yourself in other ways/by yourself, is a skill. It's during the teen years that you need to keep your kid really busy.. you know to stay out of trouble.
I have always thought learning an instrument to be important because it stimulates the brain, but if your daughter really doesn't like it, I wouldn't force it just because it is good for her. If she is musical, she will show interest for an instrument later on as she grows up (perhaps for a different one).
I wanted to play piano as a child, but we had a classical guitar lying around and no money for a piano, so guess what instrument my parents made me learn when I showed a musical interest? I liked playing the guitar because I love music, but my constant yearning for a piano stopped me from wanting to practice my guitar as often as I should.
I was naturally good at it though, so despite my inconsistent practicing, my parents kept giving me lessons for 4 years before I really got fed up and stopped practicing altogether. Not liking my teacher or the songs he gave didn't help, but anyhow, this really broke my parents because they felt they had wasted their money, and loved to hear me play.
So the moral of my story is, if she really doesn't like it, don't push her to do it. In the end you'll have invested/wasted money in piano lessons. What you could do is start by giving her 2-3 riding lessons a month instead of piano, and if she keeps up the interest you can increase the amount of lessons if you are financially able. Also, as soccer is not expensive, maybe you can do a compromise with your husband to keep the soccer and only stop the piano lessons. Soccer will help with her riding anyhow.
If you are unable to convince your husband, and your daughter really doesn't like the activities she is presently enrolled it, her increasing lack of motivation will eventually take hold of the situation. If she does find some pleasure in them, then it will be more difficult perhaps.