fortunately in my Aedui campaign i have some veteran gaesatae, botroas, gaeroas in an army so I usually just... charge...
shock and awe tactics... an army of naked ferocious barbarians chargin down on you Camillian farmers...

but rambling aside I think Sand is right. Also, the Greeks and Phoenicians tended to fight in conventional field combat with front ranks advancing, etc. so the Romans being able to rotate fresh men would allow them to constantly pit fresh troops against gradually tiring hoplite-types