The spider and beetle statuettes can be used to chase shopkeepers away, so (in most cases) you can loot the shop without being caught. As Lews said, both can also be used to get past enemies (the spider statuette makes the two army quests fairly easy, as you can walk past most of the orcs, and only be exposed briefly when you have to transform at the supply wagons or well).
I don't think anyone has discovered a use for the cat statuette. When I found one I thought it might come in handy to sneak around human territory or spy on people, but it didn't.
