One of the best reasons not to put off a brake job: worn-out pads do not just stop working, they actively damage your rotors. Waiting too long turns a cheap pad job into a costly pad-and-rotor job. Here is how worn pads wreck rotors.
Metal-on-metal grinding
Once the friction material wears away, the steel backing plate (and any rivets) contacts the rotor directly - metal-on-metal. That grinds deep grooves into the rotor surface and generates a harsh noise; see why brakes grind and rotor scoring. Every stop after that carves the rotor further.
How fast it happens
It does not take long. Once you hear grinding, the damage is already underway, and a rotor can be scored past its usable limit in a short time. That is why a squeal from the wear indicator is a warning to act BEFORE it becomes grinding; see when to replace pads.
The cost difference
Replacing pads at the right time (around 3mm) costs only pads; see minimum pad thickness. Running them to metal usually means new rotors too, and sometimes a damaged caliper - multiplying the cost. Worn pads also brake poorly, lengthening stops.
Avoiding rotor damage
Replace pads before they run out - heed the squealer, check thickness periodically, and do not ignore grinding. If the rotors are already scored, replace them as a pair with quality pads and fresh hardware.