Well heres my theory. Maybe its because that without their stitches the Intern would be a serous problem dealing with sound. Maybe an Intern without stitches could have this awful scream which can cause heads to explode or glass to break.
That could be why they have their long mouth's stitched up so perfectly and nice and tight. I think it has to deal with their voices. An Intern's voice is I think extremely dangerous.
I can see a logo saying "before the stitch 2000 regional department heads were explodeing!"