Removing slavery from any historic setting where it was extensively practised would alter the setting so much that it would defeat the entire purpose of having a historic setting in the first place.
What is the reason for using a historical setting?
Very few historical settings sourcebooks that I have read engage in any sophisticated fashion with the social and economic dynamics of the period they are concerned with. I can't imagine that a sourcebook for ancient Rome would really explain why Rome became a slave society of the sort that it did. So the reason for using that setting typically
won't be to try and model the social processes that produce mass slavery.
The following counterfactual is probably true, though also a bit vacuous: Rome would have been different from how it was had it not been a slave society. True, because being a slave society is a key feature of the actual social and economic reality of Rome; a bit vacuous, because if we think away such core elements of the society, in what sense are we
really thinking about
ancient Rome at all?
But who is qualified to assess in what way Rome would have been different? Only a handful of professional historians, and they don't all agree. (As I said, I like Finley's book. But presumably the targets of his criticism don't like it as much as I do!)
If someone wants to play an "ancient Roman" RPG, and what they're really interested in is the politics between Emperors and generals, or whether an infantry-based empire can resist the incursions of horse-mounted nomads, then I don't think backgrounding the issue of slavery is going to distort their game, or render it "pointless", in any obvious fashion.