Hard-wrapping text probably isn't something most people should be doing in most cases, especially for text documents. Computers are generally at least OK at wrapping automatically for display or editing - we've had code for it almost as long as we've had GUIs.
It's certainly not a concern for many of the places Markdown gets used - online systems where the primary form of entry is a textarea. Therefore, those places should likely not use Markdown.
What about something like Jekyll and text being edited in vim or emacs? There are many people, myself included, that prefer hard breaks for a variety of reasons.
Just because something doesn't fit your use-case doesn't mean it's not common.
There's a point to be made that if vim and emacs can't do soft line wrapping, perhaps people should be using tools that can provide basic modern conveniences like that instead of affecting the rest of us.
I never said they couldn't and they most certainly can (I simply find it awkward to work with). I've simply seen it more common to have them hardwrap than it is to see hardwrapping in a GUI editor.
You can edit with soft-wraps, I just don't like to. (I find it awkward to work with as line-movment is in lines, not soft-lines).
Also, when mixing code, you may want markdown in comments for better formatting of autogenerated docs, and you don't want softwrapping on while code (at least I don't).
