I actually haven't played far enough to see the late game yet but I find that that the production boost from clearing out dungeons makes the other gathering bonuses fairly irrelevant for anything you can get in a dungeon. I pretty much never gather wood, iron, stone, bones, meat, magic wood, or iron wood from anywhere other than a dungeon.Humans are good collectors for all kinds of metalls and woods... even bones and stuff but they aren't that good in combat, crafting or being charming.
Drows/Dark Elves are the Best Magicians, Dragonkin are the best "Normal" Fighters, Harpies are the Best Administrators...
In the Late game the race specific Bonusses make the greatest difference and can influence your play style quiet a bit.
Slaves get Malus on crafting skills but Bonusses on gathering skills, so Humans are best as slaves, Dwarfs are better of as Peon or something better once included.
Nereid grants a bonus on Fishing, Wolf girls on hunting and Dryads on farming, which makes them good slaves
Maybe Arachnes will get a Race Bonus on Tailoring and in Part 1 Scylla's got a Bonus on cooking.
The great fun was always to have a menagere of Monster girls in order to get the best results, even if all slaves have maxed abilities... but that's my personal opinion
I will have to play around with some of the late game race specific classes but I find that the regular classes work pretty well in combat overall. I could see eventually buying slaves of specific races just as dedicated maximized resource collectors though I guess. I tend to focus more initially on getting to where every new slave I get has great to excellent in all stats without needing to upgrade anything first. That pretty much makes them good at anything I need them to do.