There are a few reasons why your mechanic will tell you that it’s best to replace your bad rotors instead of having them machined. It isn’t that he’s making you buy unnecessary parts or doesn’t want the hassle of sending them out to be machined. There are some valid reasons why today you don’t have them machined.
A rotor is considered bad when it has grooves worn into it that are so deep it makes the brake pads wear out quickly. Up until rather recently, if you waited to change out your pads until they were completely worn down then you damaged the rotors, leaving grooves in it. Your mechanic would send your rotors to the machine shop and have them turned, smoothing out the surface so it is nearly brand new. That would allow you to get a long life out of your new brake pads and not get a bunch of noise when you braked. But things have changed and now they are either replaced or just left alone.
Rotors are made very thin these days and to machine them would greatly decrease their life. No good mechanic wants to charge their customer for a break job and then have them return in 20,000 miles to get another brake job. He won’t stay in business long doing that.
So, he’ll advise that you just replace the rotors along with the pads and any other parts that are worn out. Then everything is new and it’ll be a long time before you need another brake repair.
If your rotors are barely worn, he might suggest you just leave them as they are. You might hear a little minor squeaking, but it might be worth it to save a little cash. Usually, rotors are not just a little bit damaged, so this scenario doesn’t happen often.
Another issue with rotors is that these days it costs so much to machine them that you could have purchased new ones. So, your mechanic will just replace them.
Lastly, when a rotor is machined it doesn’t get that original proper finish, so it’ll make noise. And people hate it when they make a lot of noise after paying to have the brakes done.
So, be happy your mechanic opted to just replace those bad rotors.