I believe that you should be able to Own 2 houses per account with a 4 max in multiple accounts. Any more then that is Excessive and no one needs that many houses for every one of their characters that being said I believe that houses should be ACCOUNT wide as if you own one you have ownership rights across your many characters. Those who are Using this loophole should be punished as they are knowingly exploiting a loophole and cheating the system. a simple IP check could root them out and can be appealed on the case by case basis under certain circumstances.
Am I calling for the ban-hammer? No. However those who used this loophole should Lose their extra plots effective after their second plot with 0% return on their Gil as they used a dishonest method of obtaining it.