The big problem with work rosters is that they don't let you select bins, benches, or scenery-only buildings (IOW, buildings without rides or shops). It's therefore hard to make sure the border areas between park areas are covered. You CAN select rocks, bushes, and trees when creating or editing a work roster, but those don't seem to be persistent. Only rides, shops, and entertainment points seem to be saved. So, if you want to use work rosters and don't want gaps between them, you have to add entertainment points, even if you have no entertainers and regardless of whether the entertainment points fit into your overall park.
So I think there is much room for improvement. I would allow selecting ANY placeable object or building as part of a work roster. Either that, or do like the OP said and let you draw the work rosters on the map somehow. Otherwise, you'll have gaps.