I've been looking at OpenTransactions for a while and I'm still not getting why you would want to use it over a simple number in your database.
As I can figure, whenever you transfer a token, you have to register that with the central server to prevent a double spend.
If so then what's the purpose?
If your money is stored as a simple number in a database, that means the server can change the number, and thus change your balance.
On Open Transactions, the server cannot change or forge your balance.
I'm sure there are many other reasons but that seemed like a good start.