Personally, I don't see SC as so much a 100% auto-mining system, but more of an assistant for semi-manual branch mining. Its ridiculously cheap cost makes it very early-game, and its ability to keep mining as you mine the walls for resources or explore a cave it has uncovered in addition to its larger tunnel size makes for MUCH more effective manual branch mining. Plus, since you're following it all you need is one or two stacks of rails.