My answer is referring to graded stakes races, which are thoroughbred horse races in the United States:
Obviously, interference is against the rules, with or without a Jockey. Horse racing is a very dangerous sport. However, I don't believe there has never been a case in history that a race horse that lost his jockey caused a problem on the track by interfering with other horses. It's, also, fact that it is very rare for a horse to lose his jockey. These are quality horses, and even without a jockey trained race horses know where the finish line is, and will make it there by the end of the race if allowed. When horses workout on the track, they're always asked to run hard until the finish line. It is what they know, horses are very smart and they learn. Thoroughbred racehorses love to run and they enjoy beating the other horses. Horse-racing enthusiasts like to say that the jockey accounts for 10 percent of a horse’s performance on any given day. As indicated the outriders will try and catch the horse and pull him up. In any event the rule calls for disqualification if "a leading horse or any other horse in a race swerves or is ridden to either side so as to interfere with, intimidate, or impede any other horse or jockey." If the horse loses the rider after the break and before the wire, the horse is disqualified, not least because he is abruptly carrying substantially less weight and a horse must cross the wire carrying the same amount the rider weighed in at before the race. With that it's important to have a jockey. Jockey's can help a great horse win. The best jockeys know an animal’s strengths and weaknesses. Plus, the best jockeys will understand these differences in the horse they ride on, and the other main contenders of the race.