There are always going to be tradeoffs, I can easily see a shared key shared through gift wrap DMs be a good enough usecase for something like say: regular "Instagram stories",  it is good enough privacy of content. 

Privacy of metadata is also very hard to crack I. This case, but still maybe doable with chain analysis kind of stuff. 

So.. Should you sell drugs with this something like this? Probably not, could you share fun food pics on your stories with your friends that you don't want to tell the whole world about? 

Yes.