I have always looked at training fins as an item to use only during specific drill sets. I see plenty of triathletes at masters swimming that wear them the entire practice. The purpose of the fins is to give you a better leg strength workout, not simply to propel you faster. It seems that many people like them because they make kicking faster easier. Not sure how constantly wearing them in practice translates to faster race times. They also help float your feet which makes it easier to stay in proper swim position. Weaker swimmers or non-swimmers getting into the sport often have trouble keeping their legs straight with their feet near the surface. Most triathletes and distance swimmers use a 2 beat kick which is more helpful in maintaining proper body rotation than in providing massive amounts of forward propulsion. Sprinters or short distance swimmers using a constant 6 beat kick will see more benefit out of training with fins. Maintaining the rapid 6 beat kick over 1500m or more is difficult even for the best swimmers.

That is my long answer.

My short answer is keep them and use them. They are a great training tool. Use them during sprint kick sets or short distance sprint or speed sets. They will help build your leg strength making overall kicking and swimming more efficient. Just avoid using them during the entire swim workout.