Very true Checkboard,
however, have a look at our QRH (A319/320), page 2.16:
"If remaining altitude indication is unreliable:
- Do not use FPV and/or V/S which are affected.
etc....
I read this as follows:
You can not rely on the FPV all the time.
In case of erroneous Air Data I would, personally, go for the Pitch / Power settings given/memorized.
Then GPS alt.
If the FPV matches all of those, good. I _might_ consider using it.
Cheers