Just to give an example, an eBay seller sent me a message today, giving me a proposal about a global price for a list of articles. He sent the message via eBay messaging, and it arrived in my Pmail box.
When I replied with quote, only a few unimportant administrative lines by eBay were quoted, and not the message from my seller. Which is obviously what I wanted to quote. I had to go to the eBay interface online to reply with quote properly.
I think being able to use eBay messages seamlessly is a criteria for what a mail system ought to be in 2009.
And please don't tell me I must activate this and that. Or edit this and that file. It works by default or it doesn't work.