There's no guarantee that between the compiled code of helloworld.vi in 8.5 isn't radically different from the compiled code of helloworld.vi in 8.6, given the number of changes (optimizations, added/removed nodes, low-level stuff in general) between versions.
For the sake of argument, assume there is binary compatibility - those VIs won't run in any different RTE without a recompile, so what's the point? Or perhaps that simply defines that there is no binary compatibility.