Also once I use the BTC in a wallet I can essentially disregard the private key right?
You can, but most people maintain one or more keys for convenience. For example, the one in my signature is the first one I got, but I also have separate keys for receiving BTC from different sources, as a simple form of bookkeeping or to verify that a specific transaction took place. The hidden keys that your wallet uses are discarded when the money they contain has been transferred out.
Not true. The standard client
never deletes previously used keys automatically under any circumstances and has no built-in way to do so manually, and with good reason.
Once a key is deleted, if somebody sends coins to that address in the future, those coins effectively disappear forever, and there is no way to ever get them unless you happen to have a backup copy of the key lying around somewhere. And don't think that because an address is hidden in the client, nobody will ever try to send coins to you that way: a common (but dangerous, for unrelated reasons) method of performing a refund is to send the coins back to the address they came from. If you've deleted the key to that address, then your coins are gone forever.
Don't even think about trying to delete private keys from your wallet without a damn good reason. Nothing good will come of it.