You're absolutely right, Kaffe -- I was venting; just need to get things like these off my chest once in a while.
Just not very certain how to approach/reply to her. I'm thinking of writing something like this:
"Wife, your concern for E's safety and well-being is valid. I feel the same way too. This Couchsurfing organization does a great job screening and verifying travelers, but I also do my own screening on top of that to ensure that the surfers fit our home environment. I also use it as one of the opportunities to teach our son about being hospitable, charitable and just to meet people whom I think he'll enjoy having around.
Nonetheless, I will put this activity on hold until such time we can discuss its pros and cons.