new pads = new rotors? A rule?

[FTD]

I was just told that my front brake pads have about 1.5mm wear left before the sensor will tell me to get new pads. I was also told that I should replace the rotors whenever I replace the pads. I have dug around for similar statements or questions on this forum and didn't find answers, so pardon me if this is a redundant thread.
I challenged him as to why, but I don't remember what he said. It was something like seating and steering vibration, not that my rotors are worn.
The car has 22+ k miles, or something like that. Definitely less than 30k.
What say ye?