It would be cool if you would not be able to access certain troop-trees by training units from peoples other than your own.
As in, romans would not be able to create cavalry as good as germanic cavalry, that wasn't their culture.
They would hire these troops as auxiliary/mercenary troops, however, when those people were assimilated, they "romanized" and became less good on what gave then that military edge.
So the only way to get "good troops of a certain kind" form a certain people, was through not assimilating then, not influencing their culture but keeping good relations with individual tribes.