As a user of messaging services, I nearly never want to delete a message. I want to be able to use my digital memory extension (phone) to store messages so that I can easily recall my conversations. Rarely do I want to delete a message. In fact, I would only want to delete it if it's sensitive: I rarely message such sensitive things. Most people fall into this camp. It's rare for someone to never want any message to be kept.
Why do you want your messages deleted by default when you use one of these secure messaging clients?