I suppose it'd be "simple" for the device to answer the call and then prompt for a password before ringing for the user, but then the random caller needs to know the password. But then again, it could be as simple as "This is Siri, please say the name of the person you're trying to reach.", and since spammers usually don't have a name associated with the number they just randomly dialled, they'll be stumped.
If it works for you fine. But understand that different people have different needs.
At the moment, most spam callers just hang up so I never even get to the choice to answer, although I do wonder if that’ll change if the feature gets popular.
They leave a message and you call them back if it's actually important? Do you really answer every call you get?