Slaves can do all the dangerous and ugly work without throwing a big fit, don't waste up your dwarven alcohol, and can be used as vampire food without getting rid of a useful dwarf.
Once again...
Why do you assume all dwarves are useful, and all slaves are useless?
When we're talking about something that makes those other races a part of your society, then why are you going to assume the non-dwarves are the useless ones?
What happens when you get a goblin who has legendary armorer or surgeon? Are they still useless?
What about the peasant dwarf with no skills who is constantly on breaks and never bothers gaining skill, are they the ones who are so much more useful?
The whole basis of your argument rests upon an invalid assumption that the "slave" races would always be useless, and the dwarves are always going to be useful - you can already see how blatantly fallacious that claim is if you play the game at all, there's
always useless dwarves that do nothing but guzzle your booze. (And since when is booze anything that you need to worry about "wasting" - as the game is currently set up, you have no excuse for not having so much booze you're literally just giving it away to the traders simply to clear out some of your stockpiles.)