I really didn't get that either, seemed like a lame way to keep the sister alive (which was itself a cop out, just kill her already. It's not like the characters aren't interchangeable anyway, no one would notice). And I think running Jason works (for a pre-Zombie form, at least), gives him a cool crazed woodsman feel. Which is pretty much what he is when he's still human.
Also, word from the makers is that they'll try to have more creative kills in the next sequel:
http://www.chud.com/articles/article...ACE/Page1.html (near the bottom).