I had never realized that witnesses were supposed to be sequestered. In the movies they often bring out the surprise, star witness with a big flourish but I thought that was more because they wanted to keep it a secret from the other lawyers (and the audience) until the last minute.
Often they'll show a person who is also a witness sitting in the room the whole time the trial is on I guess for the dramatic effect.
I know, I know, it's a little silly to take law proceedings on a TV show for granted as real.
I think it would bug me though, if I was a witness for something, to never know what the other people were saying, however practical that might be.