Greg,
If the spindle is assigned to port 0, Mach3 doesn't automatically issue a Modbus command. The user needs to set up the Modbus configuration to get the Modbus command sent to the Modbus device. The user then needs to write a brain to store the returned value into the TrueSpindleRPM variable.
If someone wants to do threading, then yes, a index sensor into the SS is required and it can not go through Modbus due to the latencies in the Modbus protocol.
So, if the user needs to do threading, they need an index sensor to be fed into the SS, and the SS will also be able to calculate the spindle speed.
Therefore there should never be a legitimate reason to want to get the spindle speed via Modbus and at the same time use a spindle sensor into the SS (or parallel port) for threading.
Cheers,
Peter.