> uniqueness of pointers...

What I mean is for example: you get a pointer by allocating memory, then free that memory, then you allocate again and get a pointer to the same memory location. Without a tag those two pointers would be equal, even though they come from separate memory allocations. This can be a source of bugs if the pointer is also used as some sort of object id, and I know that at least I stumbled over this embarrassing problem more than once in C++ until I switched to tagged handles :)