I have some past experience working on front disc brakes,but none with wind-in rear discs .
I'd have thought that the frequency of pistons sold without the wind-in insert suggests that part doesnt often fail or wear and is relatively easy to swop over .And saves the cost of a part that is relatively expensive and complex to manufacture unnecessarily. But......
I found with brakes you are never quite sure what you will find until you are halfway through the job. I was rarely happy just to replace rubber seals . If the piston was worn/damaged or corroded enough to need replacement the bore may also be affected to some extent. Perhaps the bore could be cleaned to a satisfactory and precision finish bt this takes time and some experience.
A professional garage has the advantage of not only having experience but might have the various parts options available when required (just seals, just piston, piston with wind- in ,or compete caliper/pistons ). Either in stock, or on fast van delivery from their regular parts factor , with easy return of unused parts.
You might buy parts you dont need or the cost/inconvenience of returning them, or risk having the car off the road for days. And its not a good idea to take chances with brakes .
Often garages prefer to change the whole caliper as it can save labour time ,the parts may not cost that much more, and the result is less likely to be a mismatch and easier to guarantee. Plus customers are happier paying for something shiny they can see than a bit less for work they cannot see.
