>"MITM the content for the benefit of...why?"

To insert advertisements or "helpful" messages (https://tools.ietf.org/html/rfc6108)

They could inject malicious code or even just ads into those http assets and therefore compromise the https connection.

I also don't necessarily think that we need https for everything, but it's better to err on the safe side and if you're gonna start doing it, then you should do it properly.