I think the rules as written imply you can yank the weapon out as part of the same action. So effectively impaling weapons can still do double damage in one CA.
If the ath's check failed, yup, you do 1D4 damage from jiggling the weapon and you keep hold of the weapon if you want.
I'd allow people to simply also spend a CA to "jiggle" and twist the impaling weapon. A nice brutal way to torture a foe to death, increments of 1D4. Generally, most people wouldn't do this in mass combats as it'd be better to have your weapon free to parry and attack, and an impaling hit would probably down most normals foes.
There would be reasons to keep an opponent impaled. Say keeping a beast or Uz at reach with a boar spear, or keeping them crippled by pain, or preventing a healing spell closing the wound.