Mad Dummy is useful, but situational. As although it only has one health it will not be killed off by any monster with an attack of less than 4 at one time (ignoring abilities from monsters such as Astigmatism). This also means that it can survive attacks from various spells such as 'Punishment', and 'Undyne's Spears' making it quite effective as a damage sponge against Justice decks. Because of its price, it is also is decently effective in the Integrity deck, especially if placed early on against the opponent's weaker early game cards, or taunts like Jerry. Can be easily countered by silencing cards, and surprisingly well by Madjick.