Well, that sounds a little 18th century British Empire too me.
But, as much as I hate to open this can of worms, perhaps for the sake of accuracy it should be discussed:
The 'Allied' units we put in the game are based on units that, by treaty with Rome, client states were required to provide to the Roman military whenever it was required. The key words here are 'allied' (therefore allies of the state, not THE state itself), and 'client-state' (by definition, a state allowed a degree of self-rule or independence by a larger dominant state). If we accept that this is in fact the relationship that existed at this time between the Romans and a number of regions in Italy, then it stands to reason that any troops you would be able to recruit in this region would only be available IF that region is a 'client state'. If you've 'annexed' the region, it may or may not provide you with anything, and would then be dependent on your building a normal barracks\colony system and 'Romanizing' the area in order to recruit anything.
What I'm saying then, is that Rome would have access to three different kinds of units; it's own units gained thru conquest, Romanization and colonization; allied units that are NOT client state units (like Celtic , Hispanic, Greek, etc.); and client state units that are allied by treaty and thru a client state relationship.
This would mean that all 'Allied Cohorts, Triarii, Veltiles, Equites' would only be recruitable in a 'client state' building, which would give you access to them much quicker than the normal barracks, BUT, at a considerable economic penalty (currently, 40%). If you annex the region...no allied troops. If it's a client state, no Roman barracks. If you grant citizenship to the area, THEN you can build the Roman barracks, and the client state units disappear.
Not a bad idea actually, since it would provide yet another 'type' of city with only a kind of 'helper' focus, and later on in your campaign could be converted to a 'Roman' region. Anyway, I think it would make this unit more 'plausible'. Thoughts?