If radio waves use battery and not saving the mails on the device causes the devices to download them again and again, at what time does downloading them 15 times use less battery than downloading them once ? Now, think of the poor souls who have no good dataplan, fun, don't you think ?
So, while I do not like conspiracy theories, this is just plain silly, sorry, there is absolutely no valid excuse I can think of to keep it this way. I can save 1000's of emails on other non-Android ARM devices and get pretty good performance (BB10, iPhone) .
The other thing is, when I delete a mail on blackberry/iPhone, it deletes it on the server (well, it is a setting) wheras on Android, it deletes it on the device only - I guess this has to do with this limitation as well - guess what, this is bad, too.
And, finally, this k-9 alternative is not doing it for me. I cannot get it to connect to the IMAP server, it validates the password, but says 'cannot connect' (in French). Now, I've set up IMAP mail servers on UNIX so I know what I am talking about, it appears to have some problem with the encryption, from what I could see - the same settings work in the stock Android app.
Anyway, if you like to use email on a mobile device, bb10 is what you need, imho - android's not even in the same league.