I've only felt the need for a fast reload while deer hunting one time. My son (his first deer) spined the deer at about 80 yards. It was down, but you never know for how long at that point in the game not being 100% positive about the hit.
I usually keep a couple speed loaders with a Lee R.E.A.L handy because the R.E.A.L will usually load fairly easily in a dirty bore. We walked quickly closer to the deer (about 10 yards), but he was so excited he missed the second shot.
I loaded again and put one through it's head to end the drama.
I guess the speed load probably wasn't really necessary, but I wanted to be sure it was down for good as quickly as possible.