Seriously, do we need a stupid animation and a silly-looking "ka-boom" image? It just comes across as trying too hard to be cute and ends up looking childish and stupid.
Metaphors like this help people understand what's happening. If the message just vanishes that could be for any number of reasons. But with this animation it's clear that the message is being erased. Keep in kind not everyone is a hackernews-reading computer expert.
Though I am not personally a fan of cute stuff like that, if it helps in making secure messaging usable for everyone from IT experts to their grandmothers who have never touched a computer I am all for it.
