Why? It works the exact same way as Apple and Google push notification implementations do.

It requires you to have an active connection to the server all the time.

That's just an entry in a table maintained by the kernel. "Active connection" doesn't nescisarily mean data is sent over it. Also, the other poster is right, this is exactly how the other push notifications work.

Of course, but the point is that you have only one connection open (the one to Apple or GCM), not one for each app, one for each imap account, etc. If you let anybody keep a connection alive continuosly you end up with a mess.

Just like Apple Push or Google FCM. How would that work otherwise if not with an idle connection? :)

That is one connection for all apps, not n individual connections.