One of the ways the Safecoin design discourages attack is by limiting the damage a single compromised group can do. A compromised group can only steal the Safecoins it controls. Additionally, in many scenarios, someone will notice the theft.
If the cost of successfully attacking and controlling a group is much greater than the total Safecoins under control of a group, there is no financial incentive to attempt the attack.
With account credits as described, the reward from a successful attack is unlimited. A compromised group could set the credit count on any wallet it manages to astronomical levels. Additionally, no one would be able to detect this attack until people start to notice an inexplicable inflationary force affecting the currency.
Maybe you could overcome this issue by drastically limiting the amount of credits an account may hold (to say 2 or 3 Safecoin). My hunch is that it will be difficult to ensure you have plugged all the holes.