Oh, sorry, I've been really short on time recently. :s But at least the exams are going pretty well so far, and hope it remains unchanged during the whole exam. period...
So, I did check it; Impale is a kinda tricky ability, because looking at the way it works, I think it's safe to say that unlike all spells, the units are attached to the effect, and not the opposite. Because of this, you can apply all the manipulations that can be applied to models, and they'll happen to the targets as well.
All the things that you mentioned can be done via triggers too. However, I'm pretty sure if they're done via a model, it'll be more efficient, and it also enables modellers that have no coding experience to make more complex spells.
You have to keep in mind that Impale has a really annoying side-effect, that was one of the key reasons why I rewrote it using triggers: impaled units cannot be targeted while the effect lasts.