I have a 135N back .. been trying to locate a 135W ... simple answer is, if you have this back you only have to carry one camera :-) Honestly I don't use it much, except if I'm hiking or something and want to limit the load. But yea, when I shoot 35mm I do prefer my Contax. Also of course, you could have a couple of them and have them loaded with different films, something you can't do with a single 35mm SLR.
I think the "EI" backs have a locking darkslide. You can use them on the ETRS. There are the first backs, with a single release, newer models with a double release lever, and the EI's had double release and locking darkslide. I don't have any EI backs myself.
Hope this helped.