First of all, I don't want any company in transit, from my ISP, to the message broker, to the receivers ISP to mine my data and use/sell it for profit.

Secondly, there are a lot of messages which are not illegal, but can be personally embarrassing if they were to become public. Think of sexually-tinted messages, psychological help, a kid who lives in a very conservative community and has doubts about their religion, discussions about a candidate for a job position, etc.

The problem with backdoor is that the question is not if they are exploited, but when they are exploited. And this is all assuming that the organization that has backdoor access is not of ill will.