Marbits, as a non-capital side, must earn money to pop new units and pay upkeep. They most likely accomplish this via mercenary work/alliance-based salary, and mining, to which they probably receive a significant bonus. They may be able to be used as engineers, digging tunnels and the like, in addition to having tunnel-fighting skill. These advantages likely balance out their small stature and likely low overall stats.
Real world Reference
"Marbits" from Parson's world are an abbreviation for "marshmallow bits" that are a component of breakfast cereal Lucky Charms.Erf-b1-p015 When Wanda is summoning Parson, Stanley asks for someone who (among other things) snacks on gwiffons and eat Marbits for breakfast.Erf-b1-p017 Apparently Stanley gets what he asked for.
Appearance aside, marbits seem to be an abstract of the classic dwarf; able miners, living underground, and at odds with the goblins they encounter there.