Good point - there's really no reason for iMessage to have any less functionality than email.
I am disappointed there's no WiFi only setting for iMessage. Why chew up my 2gb of data when I already have unlimited SMS? I know people are excited to cut down on their SMS usage, but AT&T only has one SMS plan (unlimited) and I still use enough traditional SMS's to make it worth while. And I'm definitely not going to be turning it on and off manually; I might just wind up turning it off!