The point is that you don't need learning capability, because if you do have a source to learn from, you could  teach the Harmony directly.  Instead you need a way to input IR signals for which you don't have an actual IR signal to learn from-- for example discrete IR signals not present in the OEM remote.  
The remotes made by UEI (the Comcast is one example) can be manually programmed to send additional functions besides the ones in the OEM remote.