The Voynich Manuscript is a cipher because...?

[-Job-]

I am not using argument from incredulity.  Attempting to understand the motives of the VMS creator from the decisions that he made is not fallacious, and it would seem to me that considering how a document such as the VMS is intended to be used would be pretty important to understanding what it is and why/how it was made.  Actually I think it's a fairly standard thing to do with any document or even any artifact. I also note that you did not actually address any of the points I made.  

I interpret the second half of your original post as arguing that the cipher hypothesis is improbable because you can't relate to how or why it might have been used. I understand your reasoning in most cases but it's not a strong argument and not something that can be discussed without introducing significant speculation.

Your point about verbose cipher and word lengths i agree with and won't dispute (nor is it my job to), though it's worth mentioning that, if you accept the possibility of a cipher, then spaces may play a different a role.

The low entropy of the text is not exclusive to a verbose cipher, or at least i don't see why it should be. For example, a code book approach using roman numerals would probably have low second-order character entropy as well, even though it is the opposite of a verbose cipher.