There are certain skills people used to have, like loading a Leica III or using a metal developing reel or even estimating exposure without a meter. You can get good at these things, but I'm not sure that today a lot of people would want to put in the necessary practice when it's really not necessary. Even a 1950s camera like the M3 or M2 is a LOT easier to load and an M4 (or an M2 with the M4 quick loading kit) is even easier -- though I kind of like the standard M2 loading scheme. And Contax IIAs, Canon screwmounts, and others of the period are far easier.
I think the Leica screwmounts are an experience everyone should have at least once, but I just don't see them as working tools in the 21st century. M2's and M3's yes, but not Leica IIs and IIIs. Using a Leica III regularly is probably a lot like driving a 1950s car. It has to be something of a crusade on the part of the user -- possibly a labor of love. Nothing wrong with that, but the average photographer who wants to use a rangefinder for the pictures he will get is better off with a M. It's just as seductively well made and a HECK of a lot easier to use.