How is this different in your view from targeted ads in gmail? You say "I don't mind having ads be presented to me from past traffic" ... why? How does that offend you less? I understand the difference from a technical perspective, but from an "end user that cares about privacy" perspective it seems the same to me.
FWIW even with this off, I think Skype will still MITM you to check to make sure URLs you link aren't spammy. Messenger did that ages ago and Facebook does it too (try IMing someone a porn website for example). Not sure what Hangout/gmail's behavior is here.
Btw it seems funny to me to use the term "MITM you". It's a chat service. It has servers that route IM and do other things. Of course it's going to be in the middle of you and your friend. Now, if you're upset that one of the many things they do while your IM is in the cloud is see if they can serve an ad for it, then fine. But any chat service that isn't p2p will "MITM you" - that's the entire point.