One argument in favor of the proposal that I don't believe I've seen mentioned is that, now that we can explicitly apply types with the @ syntax, it feels very intuitive to also bind them with the same syntax. So even if this proposal does not add much technically over ScopedTypeVariables, I think it's still a good idea.
My concern is that the notation certainly suggests that you are binding types with the @ syntax, but in really it is still the type signature that guides the binding of the variables and the @ parameters just duplicate the information from the type signature.
-Iavor