Haven't we been through this? Oi.
Someone posted a good, thorough theory/explanation on how Brooke's DF allows his skeleton to function as though it were a complete body. This explanation is good in that it clears up the question of pooping, as well as how a skeleton can move or talk at all (no muscles = just dead bones). This expands to why he can punch with force (thankfully this allows for the fisticuffs dream I put out there on his introduction), as well as why using his bones as weapons wouldn't work.