Okay..
So, using only one chassis type (e.g. infantry) is good because your army will only have one weakness (concussive, slashing). If you have all three types, so a mixture of each, each weapon type can be used more effectively and have an advantage.
The stats are in the game. Concussive is 100% effective against infantry, 50% effective against vehicles and 25% effective against tanks.
Beam is 100% effective against vehicles, 50% effective against tanks and 25% effective against infantry.
Explosive is 100% effective against tanks, 50% effective against infantry and 25% effective against vehicles.
So, if you use only one chassis type, the other two weapon types are less effective against it, thus dealing less damage to your army as a whole.
I hope this helped