The pro-build is simply the most flexible build. Its not perfect but it works in the most situations. While other build usually are only preferrable in a set amount of situations they make it difficult to stay good in all situations. Damage builds have proven this by being great in even battles where the enemy and you are roughly even in your percentages.
Example: Your doing 3/0/7 build Armor:Dmg:Rng and your opponent is doing something wacky that works like 6/3/1. There build as wacky as it seems actually works in close battles where the enemy is even in the amount of anti-chasis that they are using. And it really comes down to the point in the game where tactics, activity and strategy are used to decide a round. Pulling together team mates and allies ect.
But for me I usually just do the standard pro-build b/c of its flexibility w/conquers, enemies, ect.
What is more important than your build is your ratio.
If you have 2 enemies and they are inf chasis and tank chasis a good ratio would go as follows. 40:10:60. (Consecutive:Beam:Explosive) For every 10 squads. This concept I think is overlooked more so than the "pro-build" which has become rather famous and standard