Flexible SSL/TLS is useful as it truly does provide some of the benefits of a secure connection–it certainly prevents ISP ad injection and casual coffee shop network sniffing–but its major problem is the one you've identified, which is that it's difficult to distinguish from a "true" SSL connection. I suspect the people who are using this feature are doing so because they won't be using a secure connection at all otherwise, not because they are somehow unable to set up Let's Encrypt.