At a lower level, the brain is a very good looting monster. Same with the bolt, it is good agaisnt bases where townhalls are towards the edge and are not protected. Distracting the towers with crabs/ichis and using brains to loot is a common method. Brains are looting monsters, especially because he loots 4x the amount of resources. The 4x loot bonus does not affect the takedown bonus, meaning that a pokey will steal 100,000 fom a Harvester and 200,000 as a knockdown bonus, total is 300,000, while Brains will loot (4x) 400,000 from the Harvester, and get 200,000 s the knockdown bonus, giving a total of 600,000. In this instance, the brain actualy got 2x (those nmbers are not real, they are made up for this instance )
Another note about the Brain is that he is a strict looting monster, not a damaging monster. He steals all of the loot out of the Harvester. If he steals all of the resources out of a Harvester to the point ( because of 4x resources) then he takes the knockdown bonus int he main attack. As soon as he cannot get any more loot than the Harvester holds, he leaves it and moves onto the next, not necessarily knocking it down.
Well, Brains are not good in bunkers because of thair production stats, health and damage. The Brain is not meant to be a defender.