Howdy,
The biggest reasons would be location. (south-warmer climate, north colder climate etc...) I am sure if your in North Dakota, you really do not want a newborn calf in January. In Texas, its not so much of a problem. The next reason would be bull corral location. Otherside of the farm for me. One saturday night wiff, and a fence is just an optical illusion to a bull wanting some__________. If you have a location to keep the bull, and a steer buddy (freezer beef) to keep him company. Bull's really do not like being totally by themselves. They will try to get somewhere, somehow to be with a companion.
Herd vaccination program is easier when all are ready for shots, and also for AI breeding too.
Fall calver myself.