Is it important to backup public keys for iOS Developer Certificates?
There's no need of backing up the Public Key as it is in the actual Certificate, which can be easily downloaded from the iOS Dev Center.  

When you manually generate a Certificate, you firstly need to generate a Certificate Signing Request through the Keychain Access application. When this is done, both Public and Private keys are generated. The private one is automatically saved into your Mac, whilst the public key is included in the CSR file sent to the Provisioning Authority. One of the items contained in the generated certificate sent back from the Provisioning Authority will be this public key.