Not quite sure if this answers your question, but at least on Linux, there is the x32 ABI, which uses most of the changes of x64 over x86, except that pointers are 32-bit. This allows programs to use more registers & other goodies from x64, while keeping pointers small.

If your program doesn't use over 4GiBs of memory, it can result in a smaller memory footprint (less space used for pointers) and better performance (smaller structs fit better in CPU cache).

Also, there are people who run a 32-bit OS on a 64-bit CPU and don't know better.