Hellrazor is not even using soldiers who have strength less than 60 :-)
For the rest of us, the roles don't make too much sense... unless you too play xcom rpg style (i.e. "8 starting rookies save the world"-style).
Well you can play the whole game zerging style, if you like it that way. But i garantee you, if will ever play my mod. This tactic will bite you in your ass in the lategame so hard, that you will run back to daddy hellrazor begging for mercy
Ok enough rediculed.
Well all Soldiers start with a very low str (20-40).
You can asign them roles from the beginning. Dudes with str 38+ get Rocket Launchers + Misseles. Or getSniper Rifles and so on.
Depends totally on your playstyle what you like most.
Give them a few missions, were they have some kills and they will most likely improve there Str.
I personally have my combat teams ordered and sorted and equipted with self asigned roles from the beginning of the game, latest before i start my first mission with this team. It improves combat effectivness and gives me a more fluent playstyle.
And i am lazy. I do not want to equip my teams each time before combat, this is a waste of time. Equip once, check after a few missions and load more on the soldiers who can - done. Off course new Weapons, like Lasers and Plasma require a reequip.