interesting but in that case no point in keeping the x bit either and suid binaries should just be 4700 ?

If they don't have world-execute permission, an access(2) check for executability would return negative, leading to things like shells not tab-completing it. The kernel would also deny attempting to execute it, as it is not executable for your fsuid.

  $ sudo chmod 4700 hello
  $ ./hello
  bash: ./hello: Permission denied