still not many are doing it with emails. but great point, tough we all still have to pick unknown calls here and there as we expect someone, so with the email screener it's even better, as each email has a sender.
That is the drawback - sometimes important calls come from someone not in my contacts (like an emergency where someone borrowed a phone, or a contractor trying to call you.) Still, the beauty of voice mail is that, if the caller is really trying to reach you, they will leave a message. Some random number with no message is almost assuredly spam. That model wouldn't work with email (you would need an equivalent real-time notification of an email going to a spam folder, then the user would have to decide to send to spam or not send anything.)
Anyone without caller id is also suspicious. Emails have a sender, but it is also about as reliable as a caller id (i.e. not very) when it comes to identity.