Just a quick thought: Imagine if I set up an unsecured WiFi point near a coffee shop, and modified people's Google search results to include my own advertisements.
Or what if I started an advertisement referral program? People with unsecured WiFi points could install a program that would refer Google searches to my company's servers, which would modify the results. People would get a kickback for the number of referrals they got, as measured by my servers.
You could even use the same kickback system that adware companies currently use.
I think this underscores the importance of using secure protocols like HTTPS.