. They usually give a couple of reasons, but in many cases the main

I am not a professional or even a particularly good DIY mechanic (just
for the record).  I think most shops resurface rotors to avoid 'come
back' problems with noise etc. on brake jobs.

On my own cars, I never turn the rotors unless there is a specific
need.  I know the 'theory' is to make the flat surfaces of the pad/
rotor match, and to take off any rust or other problems with the
rotor.  But even if a rotor is not perfectly flat after a bit of
driving the pads will mate to the rotor and if there were no noise
problems with the old pads there shouldn't be any with the new.  Any
time you cut a rotor you are, of course, taking off metal - why do
that if it isn't necessary?  Its not just a 'cost' issue.

I also agree with the poster who noted that not all shops use the
brake lathe that well - in fact, I think proper turning of rotors may
very well be a dying art.  In the hands of a skilled operator, a rotor
can literally be brought back to life 'from the dead' - but like so
many other things you don't just slap the rotor on the machine and
take a cut.

My opinion, FWIW.

It has been years since I let a set of pads wear so thin they damaged
a rotor. And back then I was a college student. I slapped on a set of
pads, and never looked back. I had zero problems with the brakes
despite the "groves" in the rotor. GM even says they aren't a problem
unless they are so deep the bottom is below the minimum rotor
thickness. The one time I let a shop cut my rotors it was a disaster.
I took the car with brakes in good condition. They warned me the
brakes were thin, so I let them do a brake job. When I got the car
back the brakes squealed and pulsed like hell. When I complained, they
said the rotors were no good and tried to sell me knew ones. When I
pointed out the brakes were fine when I dropped the car off, they just
walked away. A lot yelling followed. So did new rotors.

8.7 is the NASA number for airplanes. NHTSA uses 10.35 and they make
it clear this only has validity once "the water depth exceeds the
capability of the tread design to remove water." The ability of tires
to "remove water" varies depending on the original tread design, the
tread depth, and the depth of the standing water. The hydroplaning
formula may work pretty good for heavily worn ties and relatively deep
standing water, but it clearly is just an approximation and can't be
blindly applies. Once again, do the calculation for run flat tires
with the pressure removed. Blindly using the formula implies the car
would hydroplane at 0 mph. I have a number of cars that have
recommended tire pressure of 30 psi or so. Using your preferred
formula (8.7 times the square root of the tire pressure) implies a
hydroplaning speed of 48 mile per hour. I've driven many miles in the
rain at speeds much greater than 48 mph and never hydroplaned. Using
the NHTSA formula gives a more realistic speed of 57 mph, but I've
also exceeded that many times. To be honest, I am not talking about
driving on roads with several inches of water, more like driving
through puddles with 1/4 inch of water. And with tires that still have
significant tread.

Yes they do. And your point would be?

Ed