I think one of the biggest factor of Whatsapp’s popularity is that sender of the msg knows (in most cases) whether (and when) user has received/read the msg or not. It gives him confidence, and in some cases, proof, that the user has read the msg.
It helps to clear the doubt from the minds of the both- sender and receiver of the msg. (because receiver doesn’t have to send acknowledgement to the sender that he has read the msg and thus now knows)
If someone would want to show that he has read a particular msg, even when he has not, it is to his own disadvantage I think.
Note: I know, I may not even know even 1% as you all guys do. But since this is all about sharing one’s thoughts, howsoever small, I couldn’t help myself but chipping in.