It would be very useful to be able to overload macro definitions with different number of/types of parameters.
Many, many times there is aneed to slightly tweak an existing macro to give it new capabilities. Rather than create a new one with a different name, it would make it much neater if you could create a new one with the same name.
Another way to achieve similar results would be to allow a variable number of parameters. Although not quite as formal, this would provide a reasonable alternative.