Koz, I think it's important to remember that when you Know exactly what someone has in their spellbook, of course their weaknesses will be obvious. goofy can still put down an enchantment on a creature (that he's put a Death Link on as well, say), not flip it over right away, and bluff a Nullify. In a game you won't know if he has one or not.
That said, I do agree that having a Nullify or two (and a Dispel or two) in your Warlock spellbook is a very good plan. It's much cheaper to have the Nullify than to re-cast the Lash of Hellfire over and over again. And what if you're up against another Warlock, who drops an Agony on you?
I guess I could post my list.