I've been thinking... in actual legend, rather than story, Unicorns really didn't do all that much. Their horn was considered a curative and a purifier against illnesses and poisons, but otherwise, they were more important as a symbol of purity than actually having much magic.
So if they kept to actual legendary unicorns, the most they would do is de-spoil food, as anything else requires another pack and we all know the problems with DLC for DLC. (Food poisoning is in Dine Out, Fugu poisoning in City Living, Illness is in Get to Work, and environmental poisoning is in Eco Lifestyle.)
I know in a lot of stories they do more, but real medieval Unicorns weren't actually that mystical, probably because they were considered "real" rather than mythic.
I'm going to hold out hope that getting access to the horn requires breeding a specail trait, rather than finding an add on accessory, but other than that, I think uncorns not doing much is perfectly in keeping with their medieval traditions.