What exactly would be the function for non dwarf inhabitants? How would they differ from dwarves than by name? What would their role be in society? How would you integrate this feature so that it's not just another trivial feature without much significance or purpose alike.
New inhabitants could get stuff off of high shelves.
Heh, they could also be launched out of catapults. They'll be the new dwarven trained paratroopers.
In all seriousness, I would be fine with it if it was a "trivial" feature, just cause i think it would be cool to nurse a growing snakeman population rather than murder everything in sight. And anyway, isn't 90% of the stuff done in dwarf fortress more or less pointless? If people only did practical things they really wouldn't have much to do here.
Now to make it not so trivial, you could make it so that gaining new citizens could add knowledge to your fortress, allowing you to tame new animals and make new weapons (like blowguns, for example). Different races could also have different task completion speed and exp gain rates. Could also make them differ in needs, many beastmen probably won't be used to a life of 24 hour booze.
Also, their basic racial traits could be useful in some situations. Having citizens that don't need clothes due to fur or even could be sheared might be cool. Citizen with strong innate attacks such as some of the poisonous bites could also be interesting additions to your military force.