What happens if you don't go online for a month, for example? If your balance is floating around in cyberpace, is there a timelimit where it can just get lost, swamped in the ever changing, growing data?
I assume the wallet balance is the total amount held within each of the wallet addresses or perhaps another way to look at it the multiple wallet addresses are just different ways to address the same wallet??
How can you delete addresses?
When you start your wallet, it updates your copy of the block chain and looks for transactions involving your addresses so that it can update your balance. The balance actually only exists in your wallet. The block chain stores all transactions and only transactions. To get a balance, your wallet goes through the block chain and simply adds up all the transactions.
Bitcoin-qt does not give you the option to delete a receiving address because there is no benefit, and because any coins at that address would be lost and any coins subsequently sent to that address (for whatever reason) would be lost.