Say you have a pointer to the start of an array and a pointer to a certain element and you want to turn that into an index. Not super common, but not a crazy special case either. I'm not sure why the other poster thought this was a limitation on the 68000 though as subtraction is one of the few arithmetic operations you can perform on address registers. Perhaps he/she confused the limitations of the AU (address unit) with limitations on address registers? The AU is used for calculating effective addresses and all it can do is add; however, normal add and sub instructions use the ALU even when working exclusively with address registers.
There are a bunch of other operations that you can't do on address registers (or at the very least, an address register can only be used for the effective address operand). This can be inconvenient if your code needs a bunch of registers, but not very many pointers. Not a huge deal otherwise.
There are a bunch of other operations that you can't do on address registers (or at the very least, an address register can only be used for the effective address operand). This can be inconvenient if your code needs a bunch of registers, but not very many pointers. Not a huge deal otherwise.