So I did some more research regarding Mokulele baggage rules. You do get a free "regular sized" rollerboard (22x14x9), but it can only weigh 15 pounds. You also get a free "personal item," which seems to be an unpublished benefit. If you need more luggage, the first bag up to 50 lbs costs $20. The free rollerboards don't actually travel in the cabin (much like on a small commuter aircraft).
This seems reasonable -- except perhaps the weight limit on the rollerboard. Does anyone know if they actually use a scale, or if there's any "wiggle room"? I ask because I know from experience that my carry-on can easily weigh a couple pounds over 15.
If I needed more weight, it would obviously make sense for my family to travel with one of those big suitcases that infrequent travellers often haul around. We could consolidate our belongings and check a single large bag for 20 bucks (50 lbs). But we're coming from the mainland and don't really want that hassle: we'd just rather travel with our regular carry-on luggage. That's why I'm hoping that 15 lb limit isn't rigorously enforced.