Big scallions in pho  

Don't have an answer for you, but imo this adds to the great look of a newly served bowl of pho. Makes you just want to dig in!

I think the scallions give a delicious visual to the bowls, though I don't eat them at all. Makes me want to eat some pho right about now!

Yep this is awesome pho presentation which is why many restaurants do it. I like to munch on them if they're fully cooked through. On a practical level, scallions can be dirt cheap in VN but in North America, each stalk sometime cost as much as $0.70 or more depending on the season, so it's less cost effective for pho restaurants in NA to do this all the time.