I'm not sure the predominant use case is people posting their
own secrets, so much as people posting gossip or secrets about others. I don't use Secret all that often, but I have a lot of friends who do. Most of them run in tech circles in SF. Most of their use of Secret seems to be "Heard X is happening," or "Heard Y is leaving Z," or "Heard A is working on B." This stuff isn't entrustment; it's the opposite. It's a willingness to divulge.
I find the gossipy aspect of Secret entertaining, occasionally useful, and maybe 25-50% reliable. It serves its purpose. I don't post anything to Secret, though, and I would certainly never post any of my own secrets.
I share the concerns over cyberbullying, libel, and other potentially dangerous issues with any service of this nature. I don't think banning a class of apps altogether is the right solution. More likely, it's a flagging system, combined with a very active moderation policy on Secret's part. There will come a time -- maybe it's triggered by government regulation or banning in some regions; maybe it's triggered by lawsuits in others -- when Secret decides it needs to get serious about moderation. IANAL. But I can't imagine that, in a hypothetical libel suit, Secret can mount a strong defense right now.