I wish they'd have used the original ending with Heather performing in the final scenes but what I dont get is if the spirits and Kane needed Carol Anne so badly to lead them to the light, what good was she to them if she was frozen?
"You win some, you lose some. But you live, you live to fight another day."
Good question, and could be one of the reasons that ending wasn't used. Although, it may have been because since Carol Anne refused to lead them into the Light once she was taken to the Other Side, Kane decided to freeze her. Also, the whole concept of the frozen family was actually a hold over from an earlier draft of the script, in which Bruce was shown on the inside of the bedroom fighting with the statue that had come to life early on in Pat's gallery. During that sequence (which was never shot), Donna, Carol Anne, and Scott would have blown over to Bruce across the floor (all of them unfrozen). A huge Kane head would have then appeared, "hissing out an evil wind," and covering them all with snow and ice till they became frozen. You can read that unused script segment on my site here:
Also, remember earlier on in the movie when Tangina was talking to Carol Anne through the puddle? Carol Anne was saying she was near the Light and that it was "so cold." Maybe the longer she stayed near it she became frozen.