When you need Dead Drop, it may be too late!
While every piece of information about your usage of Dead Drop is completely private and secure, the fact that you are a user of Dead Drop is visible. Similarly when you became a user should be also considered non-secure information.
It is therefore easy to imagine that if you become a user during some real-world events that you want to privately discuss with somebody else, the mere timing of you becoming a user already puts you potentially under suspicion.
The only way to preserve absolute privacy1 is to become a user before you actually need to be!