While this may be the simplest it is subject to threshold levels, voltage limits (both low and high), and communication speed. It probably does work in many cases, but there are also other cases where having a high impedance series resistor may not work (due to speed), or having a low impedance shunt resistor may also not work (due to loading constraints). So yes, it is simplest, but it has the potential of needing it to be tailored to the application. And on a high speed communication bus it is very likely to fail. But like everything else, you get what you pay for. But you are correct in terms of simplicity, for low speed simple bi-directional comms the resistor divider solution is truly hard to beat.