user350: Any Bitcoin wallet holds several bitcoin addresses. Such addresses are either generated manually from the GUI addressbook or automatically collecting the "change" of a "Send" transaction. The total balance is the sum of the credit of all addresses.
Why does the Bitcoin software not tell me ADDRESS1 holds 1 BTC while ADDRESS2 has 5BTC? 
The standard Bitcoin client automatically selects an address to send from. (From the comments, I learned that there exits a patch, which allows a user to choose an address to sent from.)
Does the behaviour of the standard client described here have any advantages over letting the user choose? 

user105: Having different addresses in the client is intended for increasing anonymity and for you to be able to identify different senders. It is not intended to be used as different "accounts". The concept of accounts actually does exist in bitcoind but it is not currently used in the client. It wouldn't be too hard to add, so it is possible that it will be added in the future if there is demand for it.
Normally a user is not interested in the details of specifying what address to send funds from. The client does a good job of choosing what input addresses to use and will for example use older ones first in order to reduce fees.

user403: The balance of an address is almost always useless, since money is sent from random addresses.

350: <p>Any Bitcoin wallet holds several bitcoin addresses. Such addresses are either generated manually from the GUI addressbook or automatically collecting the <a href="https://bitcoin.stackexchange.com/questions/1629/why-does-bitcoin-send-the-change-to-a-different-address">"change"</a> of a "Send" transaction. The total balance is the sum of the credit of all addresses.</p>

<p>Why does the Bitcoin software not tell me ADDRESS1 holds 1 BTC while ADDRESS2 has 5BTC? </p>

<p>The standard Bitcoin client automatically selects an address to send from. (From the comments, I learned that there <a href="https://bitcoin.stackexchange.com/questions/437/how-can-i-find-out-the-address-my-payment-will-come-from/453#453">exits a patch</a>, which allows a user to choose an address to sent from.)</p>

<p>Does the behaviour of the standard client described here have any advantages over letting the user choose? </p>
 105: <p>Having different addresses in the client is intended for increasing anonymity and for you to be able to identify different senders. It is not intended to be used as different "accounts". The concept of accounts actually <em>does</em> exist in bitcoind but it is not currently used in the client. It wouldn't be too hard to add, so it is possible that it will be added in the future if there is demand for it.</p>

<p>Normally a user is not interested in the details of specifying what address to send funds from. The client does a good job of choosing what input addresses to use and will for example use older ones first in order to reduce fees.</p>
 403: <p>The balance of an address is almost always useless, since money is sent from random addresses.</p>
