A JW will always rationalize the errors of WT leaders by arguing that they are simply imperfect men, who have never claimed to be spirit-directed. Let's assume, for argument's sake, that that's true, that they've never made such claims. The question then becomes, "Why aren't they?".
After all, JWs believe that the bible is the infallible word of God, despite the fact that, it too, was written by imperfect men. The reason for its inerrancy is that the writers were inspired by holy spirit. If the JWs are truly God's chosen religion, the means he is using for humans to gain salvation, why wouldn't his spirit direct WT leaders in the same way?