I get the whole being honest with your reviews of guest thing. But the bottom line is that the guest reviews of the host will make or break a host. Good reviews will put your listing closer to the front of the line in terms of when your listing comes up in searches, your bookings will go up because of people reading them, plus they give a host the feeling of a job well done . Bad reviews will do the exact opposite. That said, currently AirbnB allows the guest to read the host review before submitting his own. By doing this a host will only be hurting himself if he gives an honest review. If a host gives a guest a bad review the guest is going to give the host a bad review, even if that means lying, its how they are going to defend themselves. For example say a host says in their review that a guest left the place trashed, didn't throw their garbage in the garbage, spills that wasn't cleaned and allowed to dry. The guest will read this and will want to defend themselves, so they put in their review how the place was dirty when they got there, and that their was stuff dried up on the counter and how they couldn't throw their garbage out because the garbage can was full. Even though they were lying people wont know that. They wont know that the guest review came after they read yours. Its going to look like the guest was in the right. And if you dispute it in the response you get, it will at best be a "he said she said" situation but you could come as being combative and petty.
So by you giving that guest a bad review, the guest at worst has to spend a few more minutes searching because he got turned down by a host. But you the host will get a bad review, and as I am sure everyone knows 1 bad review can be devastating, especially if your new and dont have many reviews. It will cause your rating to go down a lot and will be a difficult long process to bring that rating back up. In my first four reviews as a host I received 2 five star, a 4 star and a 3 star reviews. My rating went down to a 4.3, since then I have received 17 straight 5 star reviews and my rating just went up to a 4.9 on the last one. Also that 1 bad review will never be taken down so future guest will be able to read it as long as your hosting.
I know this was a long post and was difficult to get to the end but I think I have shown that under the current system AirBnB has that giving a guest a bad review as noble as it may be can only end bad for the host.