Fetching a weather forecast reveals that I was in a certain location. Depending on circumstances, that could easily be incriminating evidence. Yes, Dark Sky should be HTTPS by default.
This is pretty deep down the threat modeling hole though, and no, normal people don't need to care about this.
HTTPS is useful, and it should normally be the default for web traffic potentially crossing any untrusted network, but let's keep it in perspective. It's an application layer protocol. It can't protect you against attacks at lower layers.
Remember Upside-Down-Ternet? [1]
But who is going to set up a hostile router in an office of five people? Or in someone's home?