The great thing about fishing is that there are rarely any definites. In this case - anglers can catch fish on sps with or without a loop. With small plastics - especially when fishing for bream, whiting or trout in clear water conditions during the day with light leaders and lightly weighted jig heads, a loop could be more visible and affect potential max casting distances. The action of these small presentations are rarely improved with a loop. A snug knot on the eye and trimmed well may draw a few more stikes. On the other hand a loop with the same lure, tackle and conditions for flathead may give you a little area of heavier trace. With heavier weighted jig heads holding larger plastics - a loop knot often improves their range of movement in deeper water. The secret in tying a loop is to practice making it as small as you can. These are more affective than large loops in getting the best out of a plastic's action. There are about a dozen loops that can be used. I personally prefer a fixed loop and my favourite is the re-threaded figure 8. However, most of the others are good too.