Question: What is the point of including a weakness like the crotch vulnerable spot on your robot? Ivan opened himself up for that one.
Question: What is the origin of the magic cupboard?
Answer: The cupboard isn't magical. It was found by Omri's brothers in the alley behind their house and given to him for his birthday. The magic comes from the key, which Omri's mom gave to him after seeing the cupboard. It originally belonged to either her grandmother or aunt, I forget which. In the books, Omri is able to use the key in other objects (like a trunk) proving the key has the magic, not the old cupboard.
Answer: There's never a "point" to including a weakspot, by definition they are a flaw in the design that is either unforeseen or unavoidable. Maybe they could not find a way round it when building the robot, or any solutions simply created other weakspots.