The way i rule it, which is the way i have always understood it, AOE spells unless otherwise stated, is one damage roll for everything hit, and you roll the amount of dice equal to the x value xdy+z

For magic missile you roll for each missile created, so a d4 for each missile fired.

I get the argument, i think its a dumb one. I saw crawfords sage advice, which honestly to me doesnt sound right. He refers to pg 196 about damage rolls which state you roll once for a multiple target roll, but if magic missile is effect one target, it doesnt matter if they hit at the exact same time or separately, its still the same single target, so you should roll however many dice you need, not multiply the first roll (which i think is even the only time we have a spell that is even worded like magic missile where it says add a d4 instead of specifically roll a xd4)

In anycase, both RAW and RAI as far as i am concerned follow my logic, but its weird, and poorly worded in this specific case.