I'm considering a spear-wielding build, and the impaling critical feat seems very appropriate from a flavor perspective, but it's effect seems almost worthless. It says an impaled creature takes damage equal to your weapon's damage dice (including special properties) at the beginning of each of its turns and when the weapon is removed. I take this to mean that there's no rolling, and a 1d8 damage spear would do 8 damage every time. That's nice, but I don't think it's nice enough to forego all of the bonuses to attack damage (such as strength bonus and weapon enhancement bonus), as well as any attacks you could take that turn. I suppose you could immediately pull the weapon out every time you impale, essentially adding a bonus to your crit equal to your weapon's damage dice. That doesn't seem worthwhile to me, considering there are feats like Vital Strike that do something similar, but don't require a crit to activate.
Am I interpreting this correctly? Is there some usefulness that I'm missing?