For what purpose?
I suppose they can pay us taxes and whatnot, but this stretches us a bit thin and saddles us with certain responsibilities.
For LOLEEVIL purposes, the villagers can give us human targets for Rin and co to practice, lab asistant/test subjects for Zayan, food for the slimes and cheap workers.
Now seriously, the Erise village is bound to have enough people of all ages and sizes to provide the human contact that the twins, Zayan, Rin and even Erdrick himself need to not to go fully bonkers (and Rin still has to attend these "human anatomy" classes before attempting to do another of her brilliant plots). They also have a blacksmith, who could provide us with an effective and comfy armor for Mercant (once he gets tired of running around in circles and screaming upon realizing his true nature), along with better gear for the gobbos (who mostly have scavenged stuff). There's also that priest who we still have to pay a visit to, perhaps he can tell us some interesting tidbits. Lastly but not least, the villagers clearly use some kind of super-vitamins to have such strong livestock and children.
Of course, Grahferde is not big enough to fully absorb the territory yet, but trading and recruiting is something that can be arranged. Besides, that way they would no longer have to fear more bandit incursions.
I am afraid they only know the information relevant to them. It would not be surprising if the extent of their knowledge was that angels should never be attacked because nothing slimes have - chemical weapons, mostly - work on them.
For the infodump purposes, there were dryads.
Slime Lords surprisingly also have bullet-hell tactics as part of their weaponry, but I'm sure Erd could have survived the attack even if he hadn't dodged. Nevertheless, I don't believe Zay tricked us, she specifically mentioned that slimes "would help you to better understand your bodily functions", which they kinda did. Perhaps she wasn't aware of Angels' extra-strong physiology (which she will take as a challenge once she learns about it, much to Mercant's dismay), or she fully believes herself to be able to concoction something powerful enough to overcome even an Angel's natural resistance. In either case, whatever she ends up inventing should be considered a potential WMD hyper-toxin.
That reminds me: assuming Kek wasn't lying about the other Dark Lord's generals, following JRPGs' tropes I'm sure the remaining three are classified according to their color scheme and general theme. The most powerful of them (the so called "the Dragon") may be an actual dragon himself. Nevertheless, they all share a tendency to monologue at the first given opportunity and severely underestimate their enemies.