Does leaving your wallet unencrypted mean that all the private keys bundled in the wallet are susceptable to loss of confidentiality? I noticed that multiple private keys can be imported into a wallet.  If each private key originally had their own private password password to protect the elliptic curve private keys, does importing them into the wallet mean that they are clear text within the wallet and only the wallet's password is the means to protect those keys when the software wallet client is functioning?

Here is a use case for bundling numerous private keys together within a wallet. A small businesses will want to have a public key released to each of their clients.  Then, they will want to bundle the associated private keys into a single wallet.  This will allow the small business to not expose their overall income, but still spend their currency as if it where all from one pot.  Would deterministic wallets (not Bitcoin-Qt nor Multibit) be a better fit for small business need?
When you export a private key, you are essentially just exporting a plain text private key (you have to type in a password to export is a giveaway of decryption happening). So yes, an unencrypted wallet has all the private keys plain text no matter what.