Actually guys, the MCDU THS position is used to generate a warning depending on the FAC and FWC standard installed.
Below is an extract from the MOI.
• Pitch Trim/MCDU/CG disagree:
In the frame of "take off securing", a Pitch Trim/MCDU/Center of Gravity (CG) disagree caution is implemented in FAC B0621. FAC B0621 computes a theoretical Trimmable Horizontal Stabilizer (THS) position using the MCDU CG. This theoretical position is sent to the FWC which compares it with the
actual THS position. In case of discrepancy (threshold defined in FWC logics) the "pitch trim/MCDU/CG disagree" alert is triggered. This function requires FWC standard H2F6.
In FAC STD BAM620 the alert is triggered only if MCDU and actual THS position disagree. (i.e. the FAC doesn't compute the THS position)
There are also slight differences depending on which standard of FMGC is installed.