Yes and no.
The rotor is most times held on by two or three small washer retainers, on the the lug bolt, right up against the rotor. These washers have small serrations closest to the lug bolt. They are removable and sometimes come loose and fall off when the wheel is removed. Most times a curb-stone mechanic does not have these available and so - the wheel is re-installed with out the washers.
It does not hurt anything, as long as the wheel lug nuts are properly torqued, when the wheel is installed.