I think people are confusing things a bit here. Sure, you can protect your pgp key with a password, but I don't think that adds a whole lot of security to your uploaded private keys. When you upload a pgp key to keybase, it encrypts the key again, using your keybase device key. So its double encrypted, basically.
The keybase model revolves around devices. Device keys are private keys that are tied to a particular device (your phone, pc, etc) and never leave that device (unless it gets compromised somehow). The only way you can decrypt your data on another device is by registering it using another authenticated device. These keys don't have passwords.
Its basically like encrypting a pgp key with another pgp key, and uploading it somewhere, like people upload all manner of secrets to github or s3 or whatever.
Keybase just provides an easier flow to register new devices and to import and decrypt your secrets (like via a QR code scanned by your phone, for example). Your private keys are as secure as any private, encrypted piece of data that you might send out over the wire, so long as your devices are secure, that is.
If one or more of your devices gets owned, all bets are off, AFAIK. Even if you set a passphrase on your pgp key, all it takes is a key-logger to get it. And since your device is already compromised...
This is where hardware keys win out (yubikey, etc), that require a physical touch to unlock.
DISCLAIMER: I really only have a layman's understanding of crypto.