The single device strategy is to keep the whatsapp network a network of people - no bots and no spam.
They are still trying to destroy Kik, Line and Viber. The war is not over (http://expandedramblings.com/index.php/how-many-people-use-c...) Bots and spam would be just the reason for people to switch. As it is, spam might be the only thing that can kills Tinder - and people are watching that.
Line and Viber are more fun and more full featured than whatsapp. The only thing keeping people on whatsapp is that people are annoyed by having too many messaging apps on their phone. If people get sick of whatsapp they will nag their friends to install Line. Once those friends are nagged a few times they will install it. How about when they hear that the voice calls higher quality (subjective of course)?