Yes, a paritta is a protection.
I think the protection comes to the one who does the chanting or the person listening or hearing about the paritta and its "power" is an artefact of saddha (devotion/confidence) in the Triple Gem that is triggered through listening or chanting. As for miraculous cures and the like...I keep an open mind. For more information, follow this link: http://www.accesstoinsight.org/lib/auth ... ction.html
For a long time, I believed there are some links between the mind and the body. Somatization is quite undeniable, and it is just the tip of the iceberg. The effects of paritta chanting would probably fall into this category, as you explained above.
In the link you provided, in the chapter "The Book of Protection", there is a mention of a certain Dr. Bernard Grad who showed that water poured over barley seeds by psychic healer subsequently grow better than untreated seeds...
When discussing that with the monk, I made the link with the water and threads that the monks give to lay people at the end of the service. I never participated in that because I did not understand it at that time. He indeed confirmed that they do paritta chanting on the water and threads, and thus some spiritual energy is transfered to the water and threads and then to the lay people...
I have to admit this is all quite new to me. Anyone would like to comment?