Well with blockchain.info/wallet if your password is compromised then you can simply move your funds to another address that is not compromised (hell you can create a new bc.i wallet with a better password). With having a private key that is decrypted in the browser if your password is compromised and the password protected private key is stored by the forum (I think it would have to be) then it would not be possible to protect the privacy of your PM's.
If your password is compromised, it is possible to change it. One way of doing this is to generate a random master key, which is actually the key that decrypts the PM's, and encrypt the master key with a password. So how it works is you open your Inbox, bitcointalk sends your browser your encrypted master key and encrypted PM's, you type in your password, your master key is decrypted using the password and then the PM's are decrypted using the master key. If your password is compromised you can change it, all you need to do is re-encrypt the master key with the new password, however should you ever forget the current password your PM's are gone unless you have another way of recovering your unencrypted master key. This is similar to how it works with PGP.
If the passphrase to my PGP private key is compromised (but not the private key itself) then I can simply change the passphrase to my PGP private key (I think this is possible- you could have it temporarily in decrypted format then re-encrypt it with a new passphrase (then obviously securely delete all old copies of your PGP private key).
My opinion is that PGP should really only be used for private information. Automatically PGP encrypting PM's is not a good idea, you should really only PGP encrypt PM's that actual private information. The reason for this is that if the receipient's PC is ever compromised, if they are unlocking their private key everyday to read their PM's then the malware can easily keylog them, however if they are only unlocking their PGP once every month or two to decrypt private information, there is a greater chance that the recipient will discover the keylogger before they unlock the private key. This is particularly bad because People also use their PGP keys for other purposes too like signing code, and it's generally not a good idea to have multiple PGP keys (unless you have multiple identities) as it can cause confusion, normally you should limit it to 1 key per identity.
Having the forum automatically encrypt your PM's to the recipients' PGP public key allows the person receiving the message to choose their own level of security. You are right that less people will use it if it is dependent on any third party software, however the forum can only hold people's hands so much when it comes to security/privacy.
like I said automatically PGP encrypting PM's isn't the best idea. We should only use the PGP keys for really important stuff.