"PLAIN" just doesn't describe the action wanted. "NOPARSE" is technically more descriptive, but not exactly intuitive. I've had to look both of them up in the help, as well as "[CODE rich]", whenever I wanted to use that functionality.
At the very least, I would think adding some BBCode to that "Insert" drop down would be convenient. Perhaps replace the highly useful
D) "Spoiler", "Inline spoiler" and "_ Inline code" menu items with "NOPARSE" and "Code Rich"?
As far as the text conversion is concerned, I did a quick search on "[NOPARSE]" and found only seven occurances since the year 2000. Some of those were changes within [CODE][/CODE] tags. Didn't understand that. That's not a huge problem.