Soreness may not be completely necessary. All I know is, thats the way Ive trained and gotten the best results. I always strive for that soreness, if I dont feel it, it usually means I had a half-ass workout, which I dont allow to happen anymore. I takes alot for me to get sore in any bodypart, since Im used to a good deal of abuse, so I really have to push it hard to feel it the next days. Ive always used soreness as a gauge of effectiveness. There are lots of theories on soreness. At the very least, this perspective keeps me training very hard all the time.