Why is vm closing imap session on each vm-get-new-mail?

Does anyone know why vm-imap closes sessions after retrieving mail? The "The IMAP session protocol" section in vm-imap.e describes this but doesn't indicate the motivation. There is also some discusion in the "The IMAP sessions work as follows" indicates this may be because another connection may have deleted messages.

For some reason, gmail IMAP login has gotten very slow recently, to the point that I was thinking of running a caching imap proxy on my laptop.

I have changed calls to vm-establish-new-folder-imap-session to vm-re-establish-folder-imap-session and all seems to work fine and it's much faster. I haven't experimented with deleting mail behind vm's back yet.

Any insight into this mechanism would be helpful.