Hi, Andy. I've just joined, too, so saying hello from Durham.
Like you, I have found my regular machine to be quite competent with stretch fabrics. What tipped me over from "would quite like an overlocker" to "really want an overlocker" was making up a top, using some cotton double gauze, and watching it unravel as quickly as I could stitch it. I work pretty much exclusively with natural fibres and I've found my overlocker invaluable for encasing the ends of my fabric before I pre-wash it, so I don't end up with a mass of spaghetti! I'm still getting the hang of it, mind.