Exactly! I'm envisioning they getting a notification like "@2bithits wants you to sign this" then they click "Sign" and BOOM! New tx signed. Because this is a seed signer not a wallet they wouldn't even really know much about your UTXOs.

Depends if your family or friends turn on you.

You need to think deeply about all the potential problems. Perhaps friends that don't know each other & in different countries

bitcoin

why don't we use Android phones as seed signers?

Depends if your family or friends turn on you. 

You need to think deeply about all the potential problems. Perhaps friends that don't know each other & in different countries