Super unprofessional, since your job is a lot more than just saying "go." How are you supposed to give notes on changes in blocking or altered lines if you weren't paying attention when they happened? How are you supposed to respond to a crisis if you never saw it happen?
Even little things (especially little things) are important: if my actor steps on her hem and trips, she might not even remember it happened by the end of the scene, but I know to check the stitching on her bodice, to contact Wardrobe if it needs repair, ask the actor if she needs the hem raised... &c. &c. &c.
What would that SM even be putting in their reports?