Honestly, I think that saying "dwarves view slavery as unthinkable" is the wrong argument to make against this. (Every single one of these threads has that argument, and every single one of these threads has it lead straight into a "but we can mod it!" response. The argument, bluntly speaking, is ineffective, and the reason why these threads keep rehashing the same thing over and over.)
As the simplest answer to the suggestion, the proper response is, "Slavery already is in the game (for goblins), and will be a part of the "fortress" mode when goblin citadels are a part of the game, it's just that the game isn't ready for it, yet. Hence, there's no point in suggesting what is already planned, but just waiting on further developments."
As the more complex answer:
Why would you even bother with slavery, aside from the fact that you can say to yourself that you're doing it, when there are none of the social bases for slavery present?
"Racial" slavery is the exception, not the rule. Most slavery that existed throughout history was debt slavery - peasants could not pay back loans or their taxes, and as such, they (or their children, which were sold off first as property) were put into slavery to their lords or debt-holders. The reason these slaves were used was because they could be put to work more cheaply than regular workers. Even with all the guards it would take, you could get miners to excavate your ores more cheaply only paying for food and for the guards than it would take to pay free men to live in those awful conditions.
Dwarves, however? Why, exactly, would they need slaves to do their mining for them? They willingly mine. They love the mining. It's an insult to the miners to say that they'll get the slaves to do the job they take pride in doing willingly.
How are you going to save money by not paying your slaves when you aren't paying your dwarves anything in the first place, either? They're all Happy Communists.
Even the racial slavery doesn't make sense, as you don't have the sort of social or ethnic stresses within the game's societies yet to make anything like prejudices a game mechanic.
It's that the framework of the game isn't built up to actually support anything that makes the very concept of slavery anything more than a gimmick you could do just "for teh evulz".