[jlgaddis]

My SSH keys are also protected by a passphrase, but you certainly won't find me uploading them to the Internet.

[rocqua]

If it uses pbkdf2 with sufficient rounds and a decent password (say 5 words from diceware) it should be perfectly safe to upload the keys.

Better to keep them offline and keep ownership of the key as a second factor certainly, but encrypted keys can be a fully acceptable single factor.