There's no valid (and EULA-legit) reason to unlink a FFXI account. Period.
If you want to sell your FFXI account, just alter the Square Enix and POL account data to those of the person that bought it. Then make a new Square Enix account for FFXIV.
There is a reason. I quit in 2005. SE had a policy of deleting your characters after 3 months. I had a 100 smither. I did not want to sell my character nor have all my hard work deleted.
So, I gave my account to a real life friend to keep it alive. Five years went by and the account was still alive. My friend added my account as well as his to a security token. He did not expect my return after 5 years. Although I returned on September of '09
I have to text him every time, that I want to log on. We won't deactivate the token, due to the fact that his account has been hacked and stolen, and I would not want my own account to be stolen (I play on PC), also I lost my serial number (of course over 6-7 years ago) so I would be unable to recover it, if it was stolen.
If I get disconnected at 4am. I won't be able to get back on. Any events in progress will go on without me, and I just sit in the corner of my room and /cry.
SE needs to consider unlinking certain accounts. I would rather my character be deleted than sold to some random person, and have my reputation destroyed.
Dragonwarrior - Phoenix
Blacksmithing level 100+3
First NA maxed blacksmithing in Phoenix
First to create Cursed Hauberk -1 in Phoenix