Paladins offensive abilities aren't restricted to their Smite Evil attack. They have a good BAB and usually a not so bad strength, paired with the strongest defense of all classes. Thus they are too dangerous to ignore (like the BBEG could often do with a high save, high AC monk), yet not easy to drop (like some barbarians).
Or Divine Favor... it might be weak for a paladin but given the buff possibility (Let's say 50 % of all combats allow one round of preparation for the PCs) it equals out the worse strength compared to the fighter or barbarian.