My thought was that it would add some protection against a dictionary attack.

"Horse-with-a-C" is a longer password, but could possibly be hit using an algorithm that expects "with-a-[letter]" to be generated as part of a password.

The randomized misspelling of a word-or-two was intended to break a dictionary attack.

Adding grammar will make it more predictable and less secure for the same word count.

It doesn't reduce the entropy any, so I don't see how it could be less secure (assuming you don't count "with a C" as the additional words, which obviously you wouldn't).