I feel that many of these pseudo-secure, proprietary enhancements to email create a false sense of security for non-tech-savvy users. Given the smoke-and-mirrors presentation of this as a way to "secure your email^tm" and the plethora of recent info leaks, i am sure some poor c-level exec will get caught inadvertently sharing something with an external recipient thinking that it will disappear in a few days, but then…
This is definitely a useful feature to manage sensitive documents within organisations where good faith (but not necessarily diligent policy adherence) can be assumed, but it's pitched dangerously wrong as you say.