Leaving it out at a low level tends to 'infect' higher levels of code with non-constness.
It's worth noting that constness is a valid differentiator for overloaded functions, i.e. you can overload a function with no arguments to create a const version:
const Object* GetObject() const;
This is of course not valid for functions without a 'this' pointer to be made const - non-member functions and static member functions, I mean. Instance methods are where it's at.