Visually just look? We are talking tenths of a millimeter. That will make a difference. I know I'm not good enough to visually look and decide about something that is less than .004 inches (1/256 ths of an inch).
I also will imagine... since mine aren't worn halfway to the recommended replace, that replacing as soon as or even earlier than recommended will help to keep disc brakes from having issues with pistons getting a little askew in the cylinder causing rub and other issues.
Harbor Freight has very cheap digital calipers that are accurate enough for the job. Pads on my brakes come out easily for inspection. And the disc itself needs to be checked to the manufacturers spec. An under spec disc with under spec pads might be even worse on the piston. And which is in the expensive part of the brake to replace.
As to the OP's question, I can only ask why don't they do what the manufacturer of their brakes say to do for that model. Shimano, SRAM and others have documents online that tell exactly when to replace.
Last edited by Iride01; 06-02-21 at 07:52 AM.