Abstract, to me, is a necessary addition to a large team using Sketch. Without it, file management and sharing designs updates are impossible. However, if you have a small team, it's not worth the money. For us, a team of 9, it's a lifesaver. Even more so, because we are not all co-located, and ability to share a link to a specific file/artboard is a needed workaround from pointing at someone's screen.