Schema For "Other - Add Comment"
Nice question. My objection to your design is: you choose to consider user input and predefined input as the same thing (the only difference being the value of show). But they are not, in my opinion. Because predefined input are different options, but different user inputs could indicate the same idea written with synonyms, wrong spellings, etc, or could be just jokes or junk characters.

You say that this saves you from having empty strings or NULLs. But I believe that the absence of a value (custom text) is a valid use for an empty string.