Hi Pyrox.
Good questions.
As all of you know, the e-mail module is a beta. The reason is that e-mail is harder than it seems, and to have a 'state of the art' AJAXy Webmail module integrated in OpenGoo is not a trivial task. We are hoping that developers from the community jump in to help with this part. We are confident it will happen, as many have shown interest in it.
To address the 'philosophy' part of your question: when you 'categorize' an e-mail message (by that I mean: to put it in a Workspace), like with any other 'Content Object' in the system, you now share that message with everyone accessing that WS.
But, if you categorize it to a Sub-WS of the parent WS, to which only you have access (Your own WS inside the WS that all of you share), then the others will not see it.
I would like to have my personal messages automagically delivered to my personal folder as well, so I promise you that it will eventually be implemented. When do we get there? It will depend on how fast we can make the community grow and contribute on e-mail in particular.