They're the tactics I'd use if I was the legion as well. It's implied they have huge reserves of manpower, but the evidence we see in-game is that they don't have the weaponry to equip all their troops with high-tier stuff.
It makes sense to send in the cannon fodder first if you have plenty of them. They might win the battle on their own, particularly against tribals. If they don't, your more experienced troops will see exactly where the enemy positions are and what kind of weaponry and tactics they're using before moving in to wipe them out with things like brush guns or 12.7mm SMGs that we see higher-level legionaries using.
