Why not get the PNF to confirm the selection? That way, not only is e selection checked, but also a common understanding of the cleared level that was read back by the PNF. Useful in parts of the world where comms are difficult.
'Just one click' doesn't wash, as a crew should pick up on the fact they have been cleared a nonstandard level.
Not to forget mode s provides yet another level of protection, albeit not universal yet. The distraction of using hundreds I don't like.