I think the problem has something to do with Dreamhost's IMAP server not keeping mail message IDs constant. I don't know the details of the IMAP protocol, but I /think/ that each message has some sort of unique ID. Due to the Dreamhost setup, these ID values are not always constant with time - therefore mail clients end up re-syncing more information with the server than would otherwise be needed.
I've run in to this problem before trying to use offlineimap with my account. If this is real problem/annoyance for you, it may be worth email Dreamhost support to see if there is anything they can do about it, or make a suggestion in the support panel. If enough people speak up then perhaps they will do something to fix this.
In the mean time, I've been taking advantage of the fact that my gmail account now has IMAP support... though I'd still rather use Dreamhost for email in an ideal world.