Why, exactly? Is the thought that a thief is less likely to steal your laptop if he thinks it's a bag of groceries?
All of the other shortcomings seem to vastly outweigh the gain (lack of bang/bump protection; the fact that you're carrying a laptop in a grocery bag makes it more likely to naturally sway and bang against your leg as you carry it; that if you set it down like a grocery bag it will probably topple over due to lack of structural integrity; that you're leaving it out in the open rather than concealing it in your backpack; etc). And for all that trade-off, I'm not even sure what problem he's trying to solve in the first place that he feels his laptop bag should be inconspicuous above all else.