Is the scum skimming really necessary in pho broth?
It is not necessary in the sense that if you like the finished product then why do the extra work? That said, the scum that you get from boiling bones/meat isn't really the good stuff. The good stuff is soluble flavor compounds that wind up incorporated into the (liquid) broth.

Recipes from test kitchen folks (Cook Illustrated and Serious Eats) usually call for ONE skim. These usually go


Add meat/bones and boil for 15-30 minutes.
Skim.
Add spices and carry on with rest of recipe.


I can't vouch for how pho restaurants do it but in my experience one skim after the initial boil does the trick (and isn't too fussy).