Of course, if, as a defender, you lose, the attacker can only loot once over the next 24 hours, and can't attack again either over the next 24 hours. REally, the only benefit as a defender to winning is that you won't get looted by that person for 24 hours. For an attacker there are a number of disadvantages to losing (not gaining battle points, not gaining won battles, not gaining city points, possibly more troops dead or injured, possibly lower rank in a PvP ladder for the week and fewer medals). If you are feeling vindictive I suppose you can consider the attacker's loss to be your gain.
Unfortunately, due to poor AI, the game tends to be stacked somewhat in the attacker's favor. This means that generally the attacker does win no matter what defense you put up (well, there are exceptions of course, that's why I said generally). If you want to deny the attacker battle points, city points, and position on PvP ladders, often the best way to do so is to put up no defense at all. It makes you easy to beat, but the attacker gets very few points for it. He won't lose any troops or have any troops wounded, but that's often not that big a factor anyway.
Some PvPers have taken to plundering only those who put up no defense as they really hate this. So, as a defender, sometimes putting up no defense results in getting plundered by someone who would not have plundered you otherwise, even if he won.