I not only ride with one, I carry one with me when I'm out. The only time I ever had to use it was when I made a horse move for me. He didn't like it, pinned his ears and went for a chomp (right at my face level). You bet you butt he got got the crop across his cheek, chased down the pasture and back, complete with me yelling and flapping and my mare thinking it was a hilarious game (usually SHE'S the one chasing horses away.) After that he moved if I looked at him sideways. Had I not had it I might have less face than I do now ;)
I have one while riding for the occasional reminder, but I rarely need it. My legs and feet are more than sufficient for the most part, but it's come in handy a time or two. Honestly it's more valuable to me on the ground, but at this point it's habit to have it, and like I said is sometimes convenient when my leg isn't enough.