2 very good posts captain poe- i agree with the gollum subject, however id like to use that in explaining why glorfindel would have been bad for the company.
say he had joined gandalf and took down another balrog- then gandalf would never have fell, and never have returned more powerful….so it oculd be argued gandalf falling was just as important to the destruction of the ring (and the saving of gondor and rohan) as anything else
again as you say with gollum- a negative has positive consequences…..and this all comes back to gandalf saying that gollum still had his part to play in the events of the ring- basically once a ring bearer, always a ringbearer….and while a ringbearer still lives they are intwined in the rings influence…look at bilbo and frodo's reunion in rivendell after he'd given up the ring….it still had its power. gollum and sauron were both chasing it alike, and sam and frodo were both obviously very intwined in the rings power. the rest were dead…
it also shows that frodo, like bilbo, showing mercy was a good idea….and of course has the message of having mercy to your enemies, even when they betray your trust.
so this all shows that tolkien….an author claimed by some to be shallow in his writing, had many layers and levels…these people just dont know how to read it 
as for the barrow wights, i found the events pretty clear when reading it, im going through that chapter again, so its probably one of those parts that you have to read twice to get its clear meaning
rich