Interesting protocol, thank you for sharing and correcting my misunderstanding.
Uncertain-latency Not designed for RTC.
At first glance, this looks like a client broadcasts the Whisper to another server which than places it in a cache and might forward to more clients if enabled. Eventually the cache is cleared and there's no telling if the message will propagate successfully in any given time frame?
More or less it's how you described it. The current implementations broadcast messages to all connected peers supporting the protocol every a few hundreds of milliseconds.
It's also true that there is no confirmation that the message reached the recipient (in order to provide dark routing). This can be built on top of Whisper in a separate communication protocol.
Not to mention needing to buy ETH some how and than paying for every message?