Xaun Loc,
You're right that spammers never read rejection messages,
unfortunately the people whose address was spoofed as the sender get
to read thousands of those rejection messages.
That kind of Backscatter was a big problem early in the heyday of the spambots. Not so much any more: the receiving email services have generally gotten a lot smarter about handling messages with spoofed From fields.
(email)
Lena will no doubt speak for herself, but I would bet there's significance to her choice of words: "... so the *human* sender gets a descriptive rejection message..." (emphasis mine).
Shal
Shal Farley