we had a very conscientious controller in the tower who would check retractable aircraft had their wheels out when issuing a landing clearance
That's very nice, though does not solve the problem, and could introduce another one...
If you ever fly at an airport that has a tower staffed by U.S. Air Force controllers, they always make the call "check gear down" as part of issuing their landing clearance.
If a pilot gets used to being reminded over the radio to confirm the landing gear position, that pilot does not build the necessary self discipline to carry out that responsibility independently. If a controller forgets, the pilot is landing elsewhere, or any of a number of other variables, the artificial safety system is lost. And, there are GA aircraft models available as fixed gear or retractables, from PA-18, many Cessnas, right up to the Twin Otter the controller might not know if the plane is an RG or not, and thus not make the call the pilot was holing for. Finally, the controller has no way of knowing that the gear is down and SAFE. During a few landing gear problem flights, I have had controllers say to me that: "The gear appears to be down", which is fine, but that's about as much as they could say for certain.
In the mean time, flying RG planes, it is up to the pilot(s) to take full responsibility for confirming the gear position. Relying on warning systems and the such is lazy. I've trained many pilots on airplanes which have no gear position warning systems at all, and trained them to observe the gear position, and say that position out loud twice before every landing, as a matter of personal pilot discipline. If I'm on final in my retractable (which has no warning system), and I realize that I have not spoken the gear position out loud for that approach, I will go around, simply for the self discipline. I do realize that a disciplinary go around is not appropriate for the commercial world, but the discipline of saying the gear position out loud as you're checking it's position should not be a problem, and certainly can only be a good thing when there is a second pilot aboard!