Are there any advantages to an inverted serial prototocol?
One obvious disadvantage seems to be that an extra device needs to be
  included in a circuit


But if the signal needs to be buffered the 'extra' device is necessary anyway, and may be easier to implement with inversion than without. A single transistor can both invert and level-shift the signal, while protecting sensitive inputs from nasty voltages and ESD that might be coupled through the cable and external connector. Do do the same without inversion you would need two extra transistors.

Another reason for an 'inverted' signal is if open collector drivers are used, since they generally pull down rather than up. This permits (in negative logic) a 'wired-OR' configuration  with several devices sharing the wire, possibly doing half-duplex bi-directional communication on a single wire.

So why do MCUs typically have non-inverted serial UART signals? RS232 specifies the line voltage as positive for '0' or 'space' and negative for '1' or 'mark' (ie. an 'inverted' signal), but line drivers and receivers invariably (re)invert the signal at the logic interface. So the UART signal has to be positive in order to make the RS232 line voltage negative.

S-BUS actually follows the same convention as Futaba's PPM 'trainer' signal used to send analog servo position information from one transmitter to another via a cable. Hitec also uses an inverted PPM signal, but some other manufacturers such as JR and Spektrum use a positive PPM signal. Just to confuse matters even more, depending on the frequency band and manufacturer an FM radio control signal may be 'positive shift' or 'negative shift', and output either positive or negative PPM from the receiver (if it has a PPM port). So in the RC world there is no universal standard.