That's exactly equivalent to having a "size" parameter with the same size as the pointer, except you have to use a substract instruction when you want to get the length of the string, so I'd say it's inferior to just storing the length of the string.

Except that it has the important property that the (effective) length descriptor, being a pointer, would necessarily "grow" over time (across generations of machines, e.g. 16 bit -> 32 bit, etc.) and would thus never impose any artificial restrictions on string length.