'Mercenary' Units of Roman Legionaries would serve several useful purposes in the game, even if they are speculative.

1. Would represent bribed deserters, turncoats and disaffected allies fighting against Rome as a unit. It may be hard to find solid historical evidence that this ever happened, but is not hard to imagine how it might have really happened, and it is easy to imagine it happening in an alternative history mileu such as EB.

2. Would allow non-Roman conquerors of the Italian penisula access to the fighting style of the conquered people. There are many parellels- indeed the Romans themselves picked up things from people they conquered. There is no reason why that particular shoe shouldn't be able to fit another faction's foot, if you follow the metaphor.

3. Would account for the possibility of 'imitation' legionaries. I mean something different than point 2 above- what I have in mind here is Hannibal arming his men with Roman weapons even though he never really conquered the Italian penisula or eliminated Rome as a 'faction'.

4. Would also account for some of the Romans' actual historical recruitment methods. What I have in mind here is the practice, relatively common during the late Republic and into the Civil Wars, of calling retired veterans back to the colours as Evocati. Think of Pompey's boast of stamping his foot. These men were not quite mercenaries, of course, but they are also not easily represented by the normal recruitment process. They were experienced men, paid substantial bounties to return to service immediately. 'Mercenary' units would cover that nicely, I think.

If Rome were to suffer a catastrophic defeat (Hannibal attacks, burns the city, sells all the women into slavery, sows the ground with salt, chops off the hands of all the men he captures), there might very well be large bands of armed men still together with their centurions but who no longer answer to the consuls, as they have been killed. Would they just go home to their farms? Farms are gone. What would they do? Either turn into brigands or hire out.