What can I say: There is more than GMail and Exchange. Even smaller hosting companies already provide IMAP IDLE (like http://df.eu) for none or very little money.
EDIT 21-01-2015 - Announced as upcoming feature here
EDIT 20-04-2015 - Will be in 1.1.4, as stated here.
EDIT 04-05-2015 - Actually released in 18.104.22.168 for all users here