So rather than dealing with one entity, the hotel which you have to deal with, and is represented by someone standing in front of you, you have to deal with two, one who is not,
If you're a hotel dealing with one unhappy customer you only stand to lose the future business of that customer, and any negative PR (usually non-existent) that they can personally inflict upon you.
When you're dealing with a customer you get through a lucrative distribution channel you're liable to lose your standing on the distribution channel as a whole.
Negative reviews by that customer and customer service load on the distribution channel caused by you will impact your future business as a whole. This is true for all major distribution channels that I know of, such as Amazon.com, eBay and various OTA's.
I think there's a potentially big advantage to having a 3rd party involved. It can help arbitrate when there are problems. However, the OTA must really help both parties, and resolve any disputes, instead of washing their hands off it and saying they're just a messenger confirming the booking. There's little value in an OTA that only provides booking confirmation and that's it.
As far as I'm aware airbnb has made a lot of progress in this very area, verifying photos and information posted, checking the identities of the guests, that helps protecting both the host and the guest. There's no reason why OTAs can't do the same with hotels.
