ARs operate by detection of short circuits in the track power wiring, which is something I prefer to avoid. A relay connected to the switch machine insures that the polarity is correct for whatever route the turnout is lined for. In this case, only one train or engine can move into the reverse section at a time, since there's only one turnout providing access to that area. If the switch is lined properly, the polarity is lined correctly as well.
Also, a DPDT relay with a ridiculously high voltage and current rating (12 A at 240 VAC with a 12V coil) is $6 or so. Since there's never a short, there's no chance of the AR conflicting with the circuit breaker for the power district.
The reversing needs of turntables has been discussed already.