I had exactly the same reaction: good hack, where's the problem?
The solution to the problem of too much email already exists, and that solution is ... email. Email is asynchronous, and you get to decide when and what to do with it.
These guys do a good job: http://emailcharter.org
We're trying to provide a framework around that first rule of respecting the recipient's time.
It's about sending only important e-mails/"not spamming". So, regardless of whether the guy I'm sending to has an empty or an overloaded inbox, I should always send the e-mail, because it's important.
Also to your point, I'm not sure if this is a good product for workplace email exchanges. In that setting you're sort of forced to send things to people wether you want to or not.
If the sender is even thinking about the question "Does this particular individual have time to read this email?" then the chances are that this email is already far more important than average.