I have personally found that rabble combined with a spearman unit is actually as versatile as it gets due to the spearman anti cav, sure the formation won't beat two units of legionnaires, but the rabble is surprisingly competent with their maces and shields combined with the amount of them. The lowest tier troops of the other factions really can't compare. However, rabble is mostly reliant on having other units as backup, if they are alone, they will still do some damage, and could win over a legionnaire unit, however, unlike any other unit, their staying power is nonexistant, and once they start loosing, they loose hard, like not a single kill on the enemy hard, that is two legionnaire units attacking one formation of rabble, or two units of literally anything, they will just melt away. However, if they can force a one on one they are pretty good. They will still die in droves, but you can kill an entire enemy unit with them, also archers will melt in front of them in a one on one.
I think most of the peasant units definitely have a place in captain mode.