Just because it is not rational does not make it any less real. You don't negotiate with a hurricane. If you are weighing the pros and cons about renting stuff out to a friend, then there are some deeper issues involved. Is this particular friend widely known for mooching off of you? (Why is such a person your friend anyways?) Generally, if you fear abuse of your assets and personal space/time, then you also have some Jungian shadows related to abusing your friendships.
Renting something out to strangers and taking money to mitigate risk -- that seems fair to me. (But risk is emotionally related to the same kinds of issues related to renting or not renting out to a friend, or a friend abusing or not abusing your generosity).
There are ways to deal with the underlying non-rationalizable emotions that does not involve money. Hopefully, those methods will become more popular going forward.
It's interesting. I have never considered this before. Conducting transactions like this means stripping away the impersonal nature of corporations (http://onthespiral.com/unifying-value-universe), makes you much more aware of human nature, maybe gets people to start dealing with it.
Consider also http://playspent.org ... It turns out: a social net is very advantageous. (http://www.amazon.com/Dont-Need-Rise-Network-ebook/dp/B009K8...)
While I agree that perhaps in a better world everyone should simply share resources unencumbered by profit motive, the assignment of a usage fee to a couch or something does help prevent overuse of resources.
In the case of a friend sleeping on my couch, there is our personal relationship and desire not to cause future trouble that helps limit the possible risk of the couch getting destroyed--having a pecuniary barrier to entry for a stranger to use my couch seems reasonable.