Most people don't only play romans tho and with the interloper/imposter traits it's really silly if your "native puppet Celtae Vollorix" is from Pontos euxenius. Thus an answer would be welcome.

For allied generals it's indeed a bit redundant, depending on the factions policy towards Allied client kingdoms, however Client rulers really need a fitting ethnicity. I understand EB2 will use a somewhat different mechanism to deal with this.