I don't know...I used to use a single shot in my younger days of whitetail hunting, but I've been using a repeater for an awful lot of years now. There's no way I'd go back to single shot. It's great to say "put it where it counts", but where it counts to you and where it counts to the deer aren't always the same thing. Now, I don't know if the deer in my immediate area are just super deer, or what...but seems about every second year somebody in our hunting party nearly loses a deer that "was dead". I'm talking about tracking these thing through willow so thick you can't even "paddle" through them, because they just press you in place like a featherboard holds your workpiece against the router table. I've quite literally tracked deer for fellow hunters (and yes, even occasionally myself) that have had two broken shoulders, hamburger for a heart, and dollar sized chunks of lung all across a nearly 3 mile trail. This was courtesy of a .30-06 at just under 100 yards. Now, granted a second shot may not have changed a thing. After all, these deer should have been dead with the first one...but they don't seem to mind. Maybe a guy would have gut shot the thing the second time, as it was running away, but maybe that gut shot would have flopped it so that it wouldn't have the strength to get back up again?
Two years ago I shot a whitetail buck four time. I thought I had somehow missed the first 3 times, though I started shooting at only about 120 yards away. For the first shot he was facing me head on. When I squeezed, he changed direction and kept running (right to left). I got lined up and squeezed again. Again, no indication of being hit, but he turned again and ran left to right (towards the bush). I shot a third time, and he once again showed no indication of being hit, but turned and headed away from me for a few brief seconds, then turned and head right to left one more time. This last shot was at about 60-65 yards, with my .270 WSM with 130 gr. Ballistic Silvertips. By this time I was madder 'n heck, and I just could not believe what was happening. And, of course, I was starting to shake. My last shot was at about 130 yards when he turned broadside to jump the fenceline I was sitting on. I squeezed, he tried for the fence, but didn't really clear it. He just sort of belly flopped the wires, and guts went flying about 7 yards. Then he did a nice flip and lay completely still. When I skinned him out, I'll be darned if every single shot didn't expand into lung or heart (with the exception of the gut shot, of course). Luckily, the only meat I lost was a few ounces of backstrap, but there were virtually no organs left intact in that critter.
Seeing those kinds of things, I just feel I owe it to the creatures I hunt to always have at least one follow up shot. Nothing against those that use single shots, of course...but it isn't for me!