![Image](http://img716.imageshack.us/img716/3086/footholdtrappartspic.jpg)
Another use may be to hold passive mobs in place. This would be useful for keeping adult mobs next to each other for breeding, yet still allowing the baby to wander off/be separated from the adults easier.
A third possible use would be to clamp your pesky wolves down (maybe also prevents wolves from teleporting to you?). We don't want our little puppies getting too comfortable in their lush stimulation and light free enclosure, now do we?
Feel free to discuss possible ways to fit this in to the tech tree, why it's a horrible/good idea, or anything else that relates to this.