The Freakonomics podcast had a really good economic justification for Uber specifically that goes beyond what I've described here. It's worth listening to (and I can't seem to link to it directly: http://www.npr.org/podcasts/452538045/freakonomics-radio -- September 7th).
And your assessment of Airbnb of just "diverting dollars from the legacy provider" is wrong in my opinion, it turns an unused space into cash (creating a new market/enabling an underserved market), just like early-eBay turned unwanted stuff in your house into cash. I'm not fully sure where I stand on Uber, but they're certainly offering a much better customer experience than traditional taxis.
Reading the rest of your comment, I don't really think we disagree except that you seem to think AirBNB is driving additional revenue into the economy ("Also, I've stayed at AirBnb locations where there was no other accommodations options") - before AirBNB was there, what would you have done? You would have either stayed at a hotel that was further away than you'd like, or stayed home.
The consumer is definitely winning though.
I believe Sam is better at making such connections than most people I know.