So I can get the "you should only need one shot" way of thought, but I don't necessarily agree with it.
Hope for the best(I do generally expect a bang, flop), practice for and be ready for the worst, Is sort of how I look at it.
If an animal takes a hit I KNOW was good, and doesn't go down It gets shot again. If I think I made a bad shot, and the animal doesn't go down it gets shot again. If the animal goes down no matter WHAT I think happened with the bullet I get set up for a second shot, and I've used that second shot more than once.
This guy took two hits about a second or two apart and didn't react to either of them except to kick it into high gear, I'd have shot him until my mag was empty or he was on the ground, except he was in the middle of the herd at that point and I didn't have another opportunity. He died at the edge of the trees about 200yds from where the first round hit him.
Ive completely emptied my magazine and pulled rounds from my pockets on our cow hunt, and a number of other times. One little 40lb goat soaked up 2 .308 rounds and 4 rounds from my 6x47 before finally going boots up.
There are also those situations where your firing at multiple targets, which for me is almost every time were out hunting unless it's specifically for sheep.
Along those lines, If you spend time practicing with a rifle, or a type of rifle, most people can get to be pretty quick. Using a semi takes a lot of the learning curve out of it, and gives you an advantage in ergonomics from weird positions. Personally Ive never felt the NEED for semi specifically for fast follow up shots, but I do appreciate the ones I own/owned for that part of their appeal.