i try to roll with all my students round robin. exhausting if you do it 3 times a day
shen - In the past I would have just said "yes", but really there are a lot of variables to consider...
What if someone has 250 students? --is 60 years old? --teaches 5 classes a day, etc?
In general, it's probably good for instructors to train with students --at least to some degree-- but in some cases it can be a lot to ask.
As he said, so many factors to consider. After teaching multiple morning classes, kids classes, beginners class, advanced class, maybe they just want to not do bjj for a bit. Expecting them to train all of the time is ridiculous. It's like expecting the coach of a football team to get into the plays. 
It gets tough especially if the instructor has multiple private lessons everyday or weekk. Takes a toll