I have a weldment  that is symmetrical so I modeled only half and then mirrored.  In the weldment are also some sheetmetal parts.  There are lots of parts to mirror and I don't always get them all in the first go.  If I try to go back and edit the mirror to catch the missing items I get the message, "Only one body may be selected in a sheetmetal part."  Nuts now have to delete the mirror and start over again!