Still, even in a non GPS aircraft, the procedure is to enter the aerodrome ID, and align to the database ARP. It makes no difference to the navigation precision or performance of the IRS. And if you need to (ehm, ehm) modify, the technique is to slew the loaded values.
The ARP could be many miles from the gate position.
So modification of the init location from ARP to gate may be required to meet RNP, and is the SOP in that case (using slew keys) as recommended by Airbus.
why would customer opt for (if that's the case) where the GPS position can not be copy/paste into the FMC for initialisation?
I don't know for this case specifically. However, as alluded in a post above there has been a shift how we perceive GPS today vs. how GPS was perceived even 10 years ago.
Separating IRS entry from the GPS was thought of a "good thing" as theoretically they cross-check each other. E.g., to detect errors in case the GPS signal was degraded or spoofed. Automatically copying the GPS position would negate this "feature".
Today of course most of us trust a seemingly functioning dual GPS setup 100% of the time, at least until there's a major GPS-related incident in the future. Then people will complain about over-reliance on GPS and why there weren't more checks-and-balances in place.
Having said that, entering the Lat/Long manually from scratchpad was never the recommended procedure.