Serging is an overlock stitch and the extra thread isn't for strength but covering the raw edge to prevent fraying. anything over three has an extra row but it is for clothes there there is a lot of pull and movement on the seams to help secure it, where as if you just used a straight stitch they would recommend a second row of stitching (like underarms, and crotch of pants) , it is a personal preferance in my personal opinion to what you use. Sewing thread usually has a smoother finish and won't leave as much lint behind. serger thread has also come a long way since I started using it.