Multibit balance per address
All transactions are added to the blockchain. If you need to use the bitcoins associated with a single address for payment, it will refer to the particular transaction from which it has received the bitcoins. The wallet stores each transaction separate so that you can see the transaction details (txid, sender, receiver, etc). Even though it shows the cumulative value of all the addresses, you don't have to worry of how much each address stores since the software will take care of combining multiple transaction outputs if need be. It would be easier to see the combined value of each address but then it makes it tougher to store individual transactions.