unfortunately there are always the breed that don't do the ideal and somehow manage to abuse even the simplest of instructions so by overcontrolling, we know we are certain to get the desired levels with the minimum of fuss. Just this morning I got a correct readback for making FL150 by TIGER and the next transmission from me was "make the level restriction and reduce speed when able, 20 minute delay" to get the response "disregard the level restriction and pulling the speed back". English speaking LHR based shorthaul a/c. Fortunately it wasn't too busy so the added RTF didn't impact on my tasks.
Doesn't that support my argument? You can say what you want and people will still occasionally make errors. Just constantly saying an unnecessary instruction may give an illusion of security but errors will still happen. Its a bit like telling passengers to please take all their personal belongings with them. It won't stop people forgetting stuff. So just tell me the level restriction and I will make it [unless I make an error] and only give me a speed if it is required for spacing or whatever.