To me, it's just 2 different methods of fishing with bait. Both have pros and cons.
Sewing with a single hook is usually faster than sewing with a needle simply because you don't have to detach the leader from your line. You just sew and troll. Sewing with a needle you must remove the leader first, sew, then re-attach your leader. A good way to save time when using a needle is to sew up several leaders ahead. If I'm trolling for trout and salmon, I have 4 extra sewn smelts ahead, laying on ice in a cooler, at all times. When I'm trolling for white perch, I have 8 to 10 sewn smelts ahead.
One major down fall of sewing with single hooks is that you have a lot of misses. But the plus side is you get many more hits than misses, so a few misses doesn't bother me.
Finally, when using single hook sewn smelts, most often it makes a slow roll motion when trolled, which is deadly at ice-out because most lakes are experiencing their smelt spawning runs. It looks like a wounded fish and all fish, especially salmon, love it. When using sewing needles to sew smelts, the smelt does a side to side action / method, which is deadly for fishing the remainder of the year.