I'm going to disagree with some of you guys. I think one of the more valuable lessons I learned from the basic Westside template was ab work. They do theirs twice a week, not as an add on but aggressively as an equal part of the program. OK, it's not strictly ab/iso as there's more to your core than that but anyway...
Not that I considered myself 'weak' in this area, but as I was committed to do the program I wanted to give an honest effort. In my case, it truly makes a difference in several ways.
As your numbers go up, you need to be stronger to support the added weight. With compound lifts, all electrical messages are are transferred to outlying bodyparts through this region. If it is untrained, your CNS is going to be inefficient.
True, squats, DL, etc. will strengthen your torso. I think it's also true that ab work is assistence for these same lifts and will, in turn, stengthen them. It will also allow you to maintain the integrity of your lift much longer.
It makes sense in terms of injury prevention. A chain is only as strong as it's weakest link. As your numbers go up, you need to be prepared to deal with the added stress.