1. Performance:
   An additional instruction could slow down the delete performance.

2. Could it be because of const pointers.
   Then again standard could have done something for this special case I guess.

template<typename T>
void deleten(T *&ptr) {
  delete ptr;
  ptr = NULL;
}

T* const foo = new T;
delete foo;

int *foo = new int;
int *bar = foo;
delete foo;