All those dangers would encourage arming as many goblins as possible despite sieges being broken. Also, without goblins dying for silly reasons, what reason would there be to bring back slaves?
Why should goblins work when slaves can?
You bring up a most interesting point, good sir or madam.
I believe the sociological outlook of the Goblinite people is a matter of utmost importance, and is something that few contemporary Dwarves deign to publish on.
Do Goblins really see all other races as slaves, and their own "kind" as "free"?
or are all creatures just a means to an end for this pragmatic and noble race?
Would a Goblin necessarily be drawn to empathize more with her downtrodden counterparts, or, in her eyes, are they no better than the slaves themselves?
Truly, we have not entirely fathomed the depth and width of the Great Goblin Society that has stood in the shadows for so long. Perhaps, theirs is the only truly equal, and truly fair society that has broken free of the shackles of innate bias and prejudice that mar so many other races.
For if all begin life as a slave, then did not all start as equals?

I recognise that you aren't being serious, but I think it's worth noting that goblins are never intolerant:
[PERSONALITY:TOLERANT:50:75:100]
They might enslave prisoners of war, but those they kidnap are treated as equals. Of course, that doesn't mean they are treated
well, as goblins are without exception cruel and without altruism.
[PERSONALITY:CRUELTY:50:75:100]
[PERSONALITY:ALTRUISM:0:25:50]