Uhm, the TTP doesn't delegate authority to the dapp, but the user still needs to delegate a lot of her authority to the TTP. There is no "sign this token with your memo key" login, no "use this token in the memo field of a micro transaction" option, the only option the user gets to proof it's identity is using a TTP that in turn can only be used if you trust the TTP with your keys. That is a whole lot of trust to put in a TTP if all I want to do is use a few services that merely want me to prove my account ownership.