The advantage of this is obvious; if the player is weak, they won't be able to defend themselves.
The disadvantage, however, is that they don't have many points, so they don't have many resources to plunder (the reward should still be more than the cost of the units you use to attack, however). Another is that because they have less points, they will have a huge morale advantage, so even a tiny defense or wall will deal severe damage to your units. Yet another disadvantage is that other players will notice the player is inactive and farm them before you get there.
It is most definitely advised that you send a scout out to someone you want to farm, just remember this: defending scouts cannot be killed easily, only attacking scouts will deal considerable damage while the others deal only little damage to them, so if you sent a scout party and it was destroyed by the defense scouts, they'll still be there when you come back later.
Still, if you play your cards right, you can make a nice profit off of farming. Just don't forget about luck... ;)