The response rate shown on profiles is meant to show how likely it is that the member will respond to a request for a place to stay. Unfortunately, messages that are not requests for lodging also go into the feedback rate.
For example, I have recently received (multiple!) unsolicited private messages from a member who wanted to argue about some views I had shared on the forum. Anything I wanted to say, I said publicly on the forum, and I do not want to interact with this member over private message. I therefore had to answer with a single letter (".") just to keep my response rate up without encouraging him any further.
Also, one of my guests, after he got back home, wrote a long description of how his trip went and copy/pasted it into a message to all of his hosts. While it was pleasant to host this person, I'm not especially interesting in keeping in touch with him. However, to keep my response rate up, I was forced to type a few inanities along the lines of "Thanks for your message, I'm glad you enjoyed your trip." He probably wasn't even expecting a response from each individual recipient anyway.
Since private messages can include non-requests like the above, WS really needs a simple way for members to specify that a message in their inbox is not a request, and therefore can go unanswered without affecting their response rate.