Question

Why is it bad to name a variable id in Python?

Answer 1

id() is a fundamental built-in:

Help on built-in function id in module __builtin__:

id(...)      id(object) -> integer      Return the identity of an object.  This is guaranteed to be unique among     simultaneously existing objects.  (Hint: it's the object's memory     address.) 

In general, using variable names that eclipse a keyword or built-in function in any language is a bad idea, even if it is allowed.