Hmm. My reply here has disappeared, but what I basically said was that the same logic could be used as a reason to not hold mail on the IMAP server.
In my case as I'm simply running RoundCube on a personal server for the convenience of my family and myself when away from home, I would like to see the actual messages cached, as I often go back and view messages days, weeks or even months later. My ISP removes messages from the IMAP server after 45 days, whether read or not, so I will lose these messages via RoundCube if they are not cached.
As my server has a 500 Gbyte hard drive, the amount of memory required is not really an issue and I won't, unlike the IMAP server, be reading and consequently storing spam.
Also the size of the header is often larger than the message, except when attachments are involved. I can see the logic of leaving the attachments on the IMAP server, as in most cases I would only download these once, but I can see that attachments complicate the necessary code.
Gary.